It probably won’t live up to the buildup, but:
The Man Who Mistook His Wife. Please.
The ego-boosting conclusion is here.
James Michner and I corresponded for a handful of years. I had met him when he was researching some parts of Centennial and it turned out we shared a number of counties we both had experiences in. He was a wonderful letter writer. They would read almost like a diary of experiences and observations for that day. At times they were slightly disconnected, but in such a lyrical and discriptive way. He would complain he was not a good letter writer, and I read in a couple of his non fiction works that he claimed to not be a good letter writer. It was far from the truth.
I also used to correspond periodically with Baxter Black. Unfortunately we have ceased to write one another and I am not certain why. I suppose it is the regular schedule of writing other things that takes us away from writing for communication. I found him a very practical individual when it came to correspondance, always to the point and having the right word for the right situation. His style emulates his professional writing style, but is more, well, to the point.
I emailed Scott Douglas about his book Quiet Please: Dispatches From a Public Librarian. I had looked forward to the book, and started off liking it a lot…but by page ten, I was kicking the book across the room and tearing my hair about the ever-loving footnotes.
I noticed he is on the Goodreads site, so I (gently) let him know that he killed a pretty good book with all the footnotes and sidebars, and he gave me a polite acknowledgement. Apparently other people had told him the same thing.
Stephen King’s my main man, but I have no reason to approach him or waste his time. When we were on vacation last year, I did insist that we find his house and had my husband take my picture in front of it. I had my eyes shut like a giant dork, of course. I think that’s about as close to my hero as I can bear to come. (I don’t mean it, Steve, call me!)
After I read Isaac Asimov’s biography back in high school (mid-1980s), I felt compelled to write him a letter. (I had also read a few hundred other books by him.)
He wrote me back! It was a real reply, typed on a postcard (with even a strikeout correction), and signed by the Good Doctor.
Unfortunately, the postcard got lost or thrown out when I went off to college and my parents moved to a new house.
Yep, that’s a winner for sure!
RR
I e-mailed Christopher Moore, asking him who else wrote in the same vein as he does, to keep me reading while I waited for him to finish his next book. He responded back very quickly, pointing me to a reading list he keeps on his site, and (if I remember correctly), specifically mentioning Chuck Pahlaniuk.
I emailed the agent of Nell Dunn (British “kitchen sink” author - “Up The Junction”, “Poor Cow”) and received a reply back from Ms. Dunn herself.
Neato!
VCNJ~
I wrote notes to Dave Barry a couple of times and always received a quick note back. Very cool.
Quite a few, actually. Several years ago I wrote a letter to Bel Kaufman, telling her how much her book Up the Down Staircase impacted my life. I read and reread it compulsively as a kid, and as a result thought Chaucer was the coolest thing ever. Still do. She responded with a very lovely personal letter that I was thrilled to receive.
I’ve also emailed several authors through their websites and received personal responses–D.J. McHale, Tess Gerritsen, Ellen Hopkins (I wrote her to complain vociferously about the only book by her that I’ve read, and the only one that I threw onto the floor after ripping all the pages out and stomping on them in utter disgust.)
And cookbook author Crescent Dragonwagonemailed me after reading a blog post I did one day with a cornbread recipe, and ended up sending me a great cookbook to review. So that was really nice.
How did she respond?
Well, I didn’t show her the photo of me stomping on her book. 
It was a bizarre response. My email to her was polite in tone, and I asked questions about portrayals in her book that bothered me. Her response was also polite in tone, but it was sort of all over the place. First she agreed with me and then disagreed with me on the same topic, defended her stance (which of course she had every right to), and told me how she’d gotten tons of letters from people telling her how the book had helped them. I went to her website, which I think was overly busy and distracting and looked at the readers’ letters, particularly in response to that book. They struck me, in the main, as having been written by people who either had an ax to grind or who were totally ignorant about the LDS faith.
It was nice of her to respond. The one time I criticized a book to the author (at his request, the book was an ARC), the author thanked me but said it was too late to make changes. He’d completely dropped a major plot line – actually left a guy standing at the door of someone he was planning to murder, and he was never seen or mentioned again.
It was very nice of her to respond. While I don’t plan on reading any more of her books, because I find her writing style to be very annoying, I do appreciate that she took the time to answer my e-mail.
I reviewed one ARC book that needed a lot of work, but it was a self-published book and had already been printed. The guy had some really good ideas, but the execution was less than ideal. He could have used a good editor. He was crushed by my review, and sent me a letter (he refused to communicate via e-mail) defending himself. He then sent me a box of material along with another letter telling me that I was right. And then he sent me another letter with a booklet of all his reviews. ay-yi-yi!
In fact, I’ve almost completely quit accepting ARC’s from authors who use this company because so far I’ve only read perhaps two that were in good, readable condition as-is. And those two I was surprised that they were self-published, because they are excellent books that I’d recommend to anyone.
I got to review a few cookbooks, and I really enjoyed that. The difference in quality from the self-published books is definitely noticeable.
I exchanged a couple of letters with Spider Robinson in the early 1980s, but my main literary penpal was Isaac Asimov. Isaac and I corresponded for several years, and I treasure his missives as if they were made of…um… as if they were made of thiotimoline.
Elizabeth Janeway, author of Man’s World, Woman’s Place: A Study in Social Mythology. Which is one of those books where the people who know about it take it for granted as a classic, but people finding it for the first time always say “Hey, how is it possible that I’ve never heard of this book?”
I sent an email to Lawrence Rowe complimenting him on his book Tempus Fugit. He sent me back a very nice reply with some spoilers for his next book.
Pretty cool guy. http://www.lawrencerowe.com
I sent an e-mail once to George R. R. Martin that was essentially thanks for the great books. About a year later I got a reply to it. 
Really.
Which isn’t to complain about Martin’s response time; I wasn’t expecting anything back at all and there’s millions of things should take priority over replying to a nerd who said, “I liked your book!” Still feel free to insert your own joke about his writing speed here.
He was better at correspondence before the Song of Ice and Fire series took off. He was selling books from his website for awhile, from his own stock. My copy of Sandkings is a battered paperback with yellowing pages that he sold me for $7. He even inscribed it. He must have been hard up for money at some point. Not now though, nosirreebob.
Another ditto (tritto?) for Scott Adams. I emailed him, to let him know that I was about to begin work at the company that employed him when he had a real job, and asked if he had any comments/advice.
He replied “the cafeteria is really good…”
A few years ago, some IMAX theaters decided not to show a film about Volcanoes because it contained references to evolution, and they were afraid of possible protests. Roger Ebert wrote a very nice essay defending science in general and evolution in particular, and castigating the theaters in question for their cowardly decisions. He even included a link to the TalkOrigins website.
I enjoyed the essay so much that I e-mailed Roger, thanking him for it and saying that I thought it was important that prominent people who aren’t in the field of science to speak out on this kind of issue. i was pleasantly surprised when he replied with a nice note.