You might be right about “Air Raid”/Millennium. I know that the short story came out first, and then the novel, but I wasn’t quite sure where the movies script came in.
John Varley also wrote Mammoth, which I enjoyed, but I don’t think it’s quite epic.
To Say Nothing of the Dog - Connie Willis
Slaughterhouse Five by Vonnegut might join this list.
I’m not a big fan of Dean Koontz, but I did enjoy Lightning.
Great story.
Also, I just remembered A Tale of Time City by Diana Wynne Jones.
This is an excellent book, but there’s no actual time travel anywhere in it. There’s a lot of teleportation, but no going into the future or past.
Still, absolutely great read.
I love time travel books, and movies as well, so I apreciate all of these great suggestions!
I have one book I loved that I haven’t seen listed here, Time Craft. Really enjoyable read, especially if you like Back to the Future type stories.
I really liked Glimpses by Lewis Shiner. A reviewer called it the “first rock and roll time travel novel”, but it is more substantial and thought-provoking than that makes it sound. Or maybe I just liked the idea of what “might have been”.
Glimpses is excellent!
Also fun is Time Travellers never Die by Jack McDevitt.
Or there’s Behold the Man by Michael Moorcock, in which a time traveller goes back to check out the life of Jesus… Set in the same time, but quite different (includes a dinosaur!) is Corrupting Dr. Nice by John Kessel.
There’s a whole series of time travel books by Kage Baker. They get a little odd towards the end, but they’re very good. The first book is In the Garden of Iden.
I was going to suggest A Tale of Time City, but Lynne beat me to it. It’s not one of Jones’ best books, but anything by her is delightful. And I’ll umpteenth recommend The Anubis Gates and All You Zombies.
A Matter of Time, by Glen Cook, is well worth looking into. It’s a mystery, but the time travel is crucial to both triggering it and solving it.
For another Star Trek book, try Fallen Heroes, by Dafydd Ab Hugh. I don’t normally read books set in commercial universes (they’re bad so often), but a friend of mine gave me this and said, “Read it. Some of the stories written by authors who don’t create their own worlds are actually pretty good.” He was right, too.
Poul Anderson: The Dancer of Atlantis & The Corridors of Time
Kim Stanley Robinson: Galileo’s Dreams
Alfred Bester: “Disappearing Act” & “The Men Who Murdered Mohammed” Both are hilarious.
H.P. Lovecraft: “The Shadow out of Time.” Not your standard time travel story, but one of Lovecraft’s best works in my opinion.
A.E. van Vogt’s *The Weapon Shops of Isher *has some time travel elements.
Poul Anderson has an episodic book, collections of his *Time Patrol *stories. I’m to lazy to try and find the title.
Ward Moore’s Civil War time travel novel Bring the Jubilee is flawed but still worth a read.
A historian in the backwater and powerless 1950s U.S. - the Confederacy having won the war decades earlier - goes back in time to study the Battle of Gettysburg firsthand. By his very presence, he inadvertently changes the course of the battle, resulting in Lee’s defeat. The Confederacy ultimately loses the war, and the historian finds himself stuck in our timeline.
Well, that fellates with great alacrity.
Do you mean
for the historian in the book, or on its merits as a plot?
For the historian. 
I enjoy time travel books. Most time travel stories like to put a modern person in the past. People love history, and a good many time travel stories are really historical fiction.
A Connecticut Yankee in King Arthur’s is my preferred in this category. Yankee
A very different kind of novel is The time traveler’s wife. This is no ordinary fantasy in which someone is sent back to a more romantic era but a complex and bittersweet story happening in our times. The Time Traveler Wife
Another different book, a new one this time: Io Deceneus Journal of a time traveler.
A book about the past of an alien planet intersecting with Earth’s own timeline.
Io Deceneus
You’re right about that. Anybody know of any good books in which one or more people from the past visit the modern world of today? (Bill & Ted’s Excellent Adventure is the only example I can think of offhand that has any of this, but that’s a movie, not a book.)
Another movie example of this would be Kate & Leopold.
Asimov did a nice little short story called The Immortal Bard. It’s just a couple of pages of dialogue between a physics professor and a English instructor during a faculty function. The physicist is a bit drunk, so he starts talking about his secret time machine and the historical figures he’s met and introduced to modern times.
He says he tried scientists first, with Newton, Galileo, Archmedes, etc. But he found that while these guys were smart, they weren’t flexible enough to adjust well to the cultural differences. So he brought Shakespeare forward.
Said he did relatively well, and wanted to know all about how history was dealing with his work. He was amazed at all the meaning that scholars had squeezed out of words he’d often written without really thinking deeply about them at all.
But the story got horrifying to the young instructor when he was told that William Shakespeare had attended the young man’s class on The Bard. And the instructor had flunked him.
You can find the story in its entirety online, but I don’t think that’s copyright-kosher, so I won’t link to it.