How about the first one I ever read
"A Connecticut Yankee In King Arthur's Court"
http://www.gutenberg.org/browse/authors/t#a53
or Poul Anderson's Time Patrol novels.. can find at Baen Free Books
except this particular one and the others in the series are 5 bucks. In fact, you can find quite a few of the books there cheaply, like the Andre Norton ones someone else listed.
http://www.baen.com
My favorite though is still Connie Willis and "To Say Nothing of the Dog" which someone else suggested. A number of hers deal with time travel. The Doomsday Book is another, dealing with the plague in the middle ages England.
"Planet Of The Apes" by Pierre Boulle was actually a far better book than the cheesy movie it has long been.
To Your Scattered Bodies Go (of the Riverworld series by Philip Jose Farmer) is fascinating. The main char dies and winds up somewhere populated by everyone who has ever died on earth. It's different
Gordon Dickson's Time storm
Robert Adam's Castaways In Time
Julian May's The Many Colored Land
Arthur Conan Doyle's The Lost World
http://www.gutenberg.org/etext/139
Edgar Rice Burroughs - The Land That Time Forgot
http://www.gutenberg.org/etext/551
David Drake's Time Safari
R A MacAvoy's The Book of Kells
Vernor Vinge's Marooned in Realtime (bobbling forward not backward like most) Can also be found in the omnibus "Across Realtime" with a short story and another time travel novel of his.
Joshua Dann's Timeshare, easy to read escapist