Another classic that I remember reading is the 1939 Lest Darkness Fall, by L. Sprague de Camp. It concerns an American archaeologist in 20th century Rome who is in the Pantheon when it gets hit by lightning, and winds up in 6th Century Rome, where he struggles to prevent the Dark Ages from starting.
Harry Turtledove has said this book influenced him to start writing "alternative history" scifi, including The Guns of the South, which has a time travel element - 21st Century White Supremacists steal a time machine to transport AK-47s, modern technology, and military history to the Confederacy late in the US Civil War, to help the South win and change future history by preserving slavery.