What made her think you hadn’t returned the book to the library? I’d ask that and throw in a “How dare you,” but everything that come into my brain falls out my mouth.
In this situation, I would definitely respond, because the opening of her message pretty much demands it. A message that says “Don’t come talk to me about this, but bitch, bitch, bitch…” roughly equates to “I know I’m making a mountain out of a molehill, and I’d be uncomfortable ever discussing such a trifling issue face to face, but I feel bitchy, and therefore I’m going to abuse the impersonal nature of e-mail. Bitch, bitch bitch…” If you don’t respond, you can expect to catch whatever dirty laundry she feels like throwing over the wall from here on out, and that doesn’t make for a very comfortable working environment.
That said, I don’t think you should respond by e-mail. Your tone can be misunderstood in e-mail, it can be forwarded out of context to your boss if your coworker really does have a personal issue with you, and you want to avoid letting your coworker send out bitchy notes while hiding behind the wall. Go to her cubicle/office, and in a very friendly tone of voice, thank her for loaning your the book and tell her how useful it was to you. Then apologize for returning it late, mentioning that you had set it out to return it on the due date but then forgot it at the office, but that you still returned it within the grace period so there should be no problem. This should all be sincere and friendly, since you do owe her a thanks and an apology.
Don’t mention the e-mail up until this point, and there’s a good chance she might bring it up and apologize. “I hope I didn’t offend you with my e-mail, I was having a bad day that day, blah blah blah.” If so, just say “Hey, no problem, thanks again.” and you’re all set. But if she doesn’t, I would bring it up. Your tone should be obsequious and questioning, but stand up straight and make eye contact while you ask “I hope I didn’t cause you any trouble - you sounded pretty upset in your e-mail. Is everything OK?” She may apologize, she might bitch at you, or she might dismiss it as if you misunderstood her intent, so what you say next depends on her reaction, but keep it brief and don’t get drawn into a bitchfest under any circumstances. Keep your tone friendly and apologetic, but keep your body language confident. Hopefully she’ll learn that she can’t fire bitchy notes at you from behind the wall without expecting to have to discuss the issue face to face, and you will remain friendly and professional throughout.
I didn’t specify this, but I absolutely agree with Mynn, the apology should be in person. The apology is for failing to return the book by the due date, bearing in mind that the coworker put the effort into acquiring it, was technically liable for any fine, and helped Kaitlin out by passing it along to her. True, the book could have been gotten by Kaitlin and there wasn’t any fine, but still she didn’t and there could have been. And yes, coworker’s e-mail was asinine, but she did perform a small favor and it wasn’t unreasonable for her to have some concern over the book being late. Especially given the office politics described, I see no reason not to offer a suitable apology.
Snotty comments, even though probably deserved, almost certainly would not achieve any good and very likely would aggravate the situation. If there are further insults and condescending lectures, or have been already, they should be addressed with a proper talking-to, but for just this one incident I would let it lie.
I used to work in a library. After a certain period of time (a couple months maybe), they stop tracking the amount of the fine and simply write off the book. And if the book ever does show up, the person now owes the library the replacement cost of the book! (It could happen when the amount of the fine is about to exceed the replacement cost.)
That said, I’d return the book as everyone else above said. Then I’d respond to the e-mail with, “What book?”
Then sit back to watch hilarity ensue.
But then, I can be passive-aggressive that way, sometimes. 
I want to add that I don’t think the co-worker really deserved all those apologies I suggested giving. It’s just that sometimes the best way to let someone know their over-reaction to a small incident was way out of line if to make somewhat of a big deal about apologizing for it. Pretty soon the over-reactor starts to feel stupid that the “offender” keeps saying she’s sorry, over and over. It works quite well.
It sounds to me as though the reminder from the library might have caught her at an off moment for other reasons and that it tipped her over the edge; I’d just apologise profusely and sincerely, return the book and let it lie.
Dear co-worker,
Wow! I’m impressed that you have remained so calm; after all, an almost-overdue library book can be a pretty scary thing. When I was a kid, I was late with a book once and it cost me 35 cents – almost my whole allowance for the week! While I already have returned it within the library’s grace period and so incurred no fines or penalties, I fully understand why this would make you so upset. I humbly accept your well-deserved admonishment.
sincerely,
Kaitlin
(If confronted, disavow all sarcasm)
-P
If you choose to respond with an email, choose your words carefully. Otherwise, they may come to haunt you down the road. Best you follow the lead of others and if you choose to respond, be succint and to the point. Leave out anything petty, personal and unprofressional.
OTOH, what you may say to her in person, after your email response – and with no witnesses present – is up to your imagination.
I think you should go with Dogzilla’s advice. “What book?”
Completely deny all knowledge of said book. 
Act like the b*tch is crazy! HEHE! 
When you do decide how to confront the situation, I would love to hear about it! Do tell!
My library does not let you checkout a book if you have one overdue. It could be that she tried to checkout a book and was denied. I’d probably be pretty mad if I lent something to someone and because they didn’t return it on time, I couldn’t check any more out.
Waitaminit! I interpreted the OP as meaning that it had already been two days past the end of the grace period—so that it was actually nine days after the official due date. Clarification? It still sounds like an overreaction on the co-worker’s part, though. Could this have been a straw-that-broke-the-camel’s-back kind of thing? Are you (Kaitlin) typically lax about getting things done/returned on time; or has the co-worker had other recent experiences similar to this of people not returning her things when they were supposed to?
I’m the kind of guy who, if I borrow something from somebody (whether from a friend, a library, or a video store) and agree to give it back by a certain time, it really bothers me if I don’t get it back on time. It feels like I’ve broken my word. If I let someone else have it and they didn’t return it on time, I would still feel bad, and I’d be tempted to blame either myself for letting them use it, or them for making me look bad—but I’d certainly try not to be nasty about it. Maybe the co-worker is this kind of person?
She was wrong to have chastised you so condescendingly and you were wrong to have kept a library book that was in her name two days past the due date.
Both of you should be whacked with a large, limp fish. I’ll also sentence Kaitlin to a minor fish-thwack for even mentioning bringing the boss into it.
Jeez, your co-worker is damned uptight, isn’t she?
I disagree with those that said to talk to her in person. She stated pretty clearly that she wasn’t interested in talking about and I think forcing the issue would be a Bad Idea. However, a reply e-mail is definitely in order.
I take it from your OP that this e-mail was sent AFTER you had already returned the book? If so, I’d just reply and say:
Co-Worker,
It has been taken care of. I returned the book at X:XXam on 4/21/04 and there were no fines. I apologise for the late return.
–
Kaitlin
While it’s VERY tempting to take her to task for the completely unprofessional tone and wording of her own e-mail, or be equally bitch in return, it’s really not worth it given everything you said about your office dynamic. Just come here and post about what a complete and total bitch she is! I would recommend saving a copy of her e-mail and your reply just in case.
The reason I note that she should quitely make the statement to her, in person, in front of other people, is to force her to say nothing, or to let her hang herself in her bitchy way.
If you do reply via email, keep it all; may need it later. Dealt with this situation at work a few times, and a record is important. The guy walked out of my office and hollered over his shoulder (loud enough for a number of people to here) his assy comment. I did nothing. He was there 2 1/2 more years before he was dumb enough to do something similar (which was a common occurence and the reason he was highly shunned) in front of the person who hired him, who then almost immediately fired him. There were a lot of emails he sent in similar tone that could have rid us of him sooner, but no one ever did a damn thing.
I’m glad to hear Marianne found work. Sorry about that; libraries are her sore spot.
email her that you returned it, paid the fines, and give her a link to this thread.
I know this is rather petty, but I don’t like other people “screwing up” my reputation, I am quite able to do that myself. What I mean is, I don’t want any repercussions from YOUR actions on MY reputation.
e.g.
Can I borrow your car?
Sure.
… a month later I get a letter from the cops saying I didn’t pay my parking ticket …
She stated pretty clearly that she wasn’t interested in talking about and I think forcing the issue would be a Bad Idea. However, a reply e-mail is definitely in order.
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The point some of us are making is that it’s best to APOLOGIZE in person. The “please don’t offer any excuses or talk to me about it” part means she doesn’t want to hear an EXPLANATION, and I agree it would be best to go along with that unless a conversation leads to it and she then appears receptive to the discussion. That does not preclude the appropriateness of an apology. Doing it in person and in public, as Mynn outlines, is best for avoiding misinterpretation and bringing the issue to a conclusion (she has to acknowledge the apology in some fashion).
Replying by e-mail carries the risk of being misconstrued and providing evidence that could be taken out of context. I don’t see any advantage to it.
Well, I messed that up. Two sentences of the quote were meant to be in the box. My reply starts after the "
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."
If this were an issue between two friends, I’d agree that an in person apology would be in order. However, this is a work situation, and I’m a firm believer in documentation when it comes to anything weird at work. Replying with a simple “It’s taken care of. The book was returned at X time on X day. I apologise for the late return” does not leave anything open to being misconstrued (IMO), and further, if one does try to take something out of context to use against her, then she (Kaitlin) has her own solid proof of what really happened: that she received a rather nasty letter from a co-worker and she handled it tactfully and professionally.
On the other hand, if she were to talk to her in person, IMO, that could more easily lead to the problems outlined in your quote above.
I complete agree with you that an apology is in order - there’s no denying that Kaitlin was in the wrong to return the book late, however, her co-worker’s e-mail was way over the top given the nature of the “crime.”