Making History by Stephen Fry. (Who knew actors could write books that aren’t about the people they’ve slept with!?) All of his books are pretty good.
Making History centers around the idea “What if you kept Hitler from every being born?” The consequences are maybe not what you thought. I’ve recommended it to several people and have gotten the response “I will always read any book you recommend from now on.”
I have enjoyed The Eight by Katherine Neville and Outlander by Diana Gabaldon. And to a lesser extent, Timeline by Michael Crichton.
I came in to suggest the following, all of which have already been mentioned.
Another vote for Ken Grimwood’s Replay. Very good stuff.
Peter David’s Imzadi is hands-down my favorite Star Trek time travel book (and there’ve been many). I was grinning like an idiot for the last third or so.
In Audrey Niffenegger’s The Time Traveler’s Wife, the involuntary and uncontrollable time-travelling of a young man wreaks havoc upon his decades-long romance with his eventual wife. More a tragic romance than sf, but still very good.
Jerry Yulsman’s Elleander Morning does some clever things with the whole go-back-in-time-to-kill-Hitler concept. The mechanics of time travel are left a little vaugue (OK, a lot vague), but it’s a nifty story.
Robert Heinlein’s 1959 short story “—All You Zombies—” is also a must-read. Temporal paradoxes galore.
I have already/will read that seven years from now/yesterday.
For a lighter take on things, try Harry Harrison’s The Technicolor Time Machine. It involves Hollywood discovering time travel.
I’m going to do my usual for these sorts of threads and (1) second/third/whatever the recommendation of The Anubis Gates and (2) throw in a recommendation of All of an Instant by Richard Garfinkle as something to read after you’ve read four or five other time travel books and think everything has been done.
They are very good.
Also by Jack Finney but not as good (IMO) as Time and Again, but still not bad: Marion’s Wall.
Whole-hearted agreements with:
[ul]
[li]Replay[/li][li]The Anubis Gates[/li][li]The Technicolor Time Machine[/li][li]*Time and Again *(but I didn’t think the sequel, From Time to Time, was nearly as good)[/li][/ul]
I have heard good things about the first 2 (by the same person, interestingly…heyy who are you really 
). But with all due respect, I must disagree strongly with that last recommendation. One of the worst, most uninteresting books I have ever read (well almost read; finally about 2/3 of the way through I finally admitted I was wasting my time), and I love time travel stuff.
Wrinkle in Time I read as a teen and liked it but think it was best served as a book for kids/teens - but then it’s been a long time, just my recollection.
I wish I had something to add but you all have taken mine. 
Although I’m curious about some of the others, thx for the suggestions…
Since no one has mentioned it, I enjoyed Octavia Butler’s Kindred.
Definitely a must read.
Also must reads. Varley wrote a short story, “Air Raid”, and expanded it into Millennium, and then turned it into a movie script. The novel’s chapters are almost all titles of previous time travel short stories or novels, and the book assumes that you are familiar with time travel concepts.
A bit off the beaten track, but I liked The Fall of Chronopolis by Barrington J. Bayley.
Nitpick, but I think the actual order was he wrote the short story, expanded and adapted that into a movie script, and then adapted the script into a novel.
See post #14 - **RealityChuck **gives it a recommendation. I enjoyed it, a lot, but don’t think of it as a classic Time Travel book as much as an exploration of African-American identity. It is no more or less a TT book than some of the others mentioned, but it feels more serious intent vs. a cool thought-provoker like Replay.
And, per Snowboarder Bo, in movie format nothing beats Primer. Love that movie.
Oops, that’s what I get for posting at work. :smack:
The Time Traveler’s Wife - guy travels through time abruptly and winds up at different random points in his life (and the people he knows). LOVED IT!
They turned it into a movie which was pretty good too.
See post 24. I really liked the book but was disappointed in the movie, FWIW.
I enjoyed both of these very much and it was from this board I learned of them.
You might like to check out the Paratime stories of H Beam Piper. All of them except Lord Kalvan of Otherwhen are out of of copyright. I actually have them all on paperback, but downloading them and reading for free on my tablet is actually better. You don’t lose your page when you fall asleep.
http://www.gutenberg.org/wiki/Science_Fiction_%28Bookshelf%29#P