OP, was the book self-published?
If it’s a been a year, I’d just keep quiet about it. He’s gotten the message by now that you didn’t like it. Because the only correct response, as far as he was concerned, would have been to rush up to him the day after he gave it to you, enthusing loudly over how it kept you up till all hours finishing it, and exclaiming over his genius. Silence for a year? Nah, he’s got it by now.
I’d feel so mean not to say anything. I imagine that writing a book is a very personal experience that makes you feel very vulnerable. When the author personally gives you a copy, what harm could be done by saying something nice about it?
“Don’t quit your day job!”
“Yes, I read your book. I look forward to continuing to work with you!”
Probably the biggest harm is to your own sanity, when they decide that since you like their work so much they’re going to talk to you about it ALL the time, and give you early drafts to read for feedback. 
“I didn’t want to put it down.”
I plowed through a co-worker’s book and said nice things about it (the ideas were actually good, the execution less so). As a reward I wound up receiving yet another book for my review. :eek:
I had to bow out of that responsibility.
I think it is self-published. I don’t have my copy with me now; it’s still at Craig’s. When he gave it to me, I did say I had a long list of books to read so it might be a while before I got to it.
I found the font work to be quite illuminating. It has a typestyle all its own.
Well, some of my “writer” friends already know my opinion of self-publishing, but in all honesty, I’d probably just say “Sorry, it’s really not my style. I’m not into Spy Thrillers, so I can’t give you much of an opinion on it.”
If asked about the book, just smile and tell him “it was a good book”, “congratulations on your book”, and similarly vague praise. If he presses for details, just say “I have to get back to work”, “I’ve got to use the restroom”, or “I’ll see you tomorrow, I’ve gotta run”, then wave goodbye and take evasive action.
As a last resort, read the book synopsis on wiki, or if you’re really worried about your standing with this coworker, skim the book and take notes of main characters and basic plot points, and try reading the last chapter.
This is though…
Well, there’s on question I wanted to ask you. What was so bad about the book? Or what made the book bad? Maybe if you know specifically what made the book so horrible as it is, you could very politely tell him that the book is not of your style of reading, as Chimera said.