"A Princess of Mars" - Edgar Rice Burroughs. For some reason, I have no idea why, his Mars series still works. I reread them recently. The prose is clumsy, the plots are rinky-dink, but it's like potato chips.

One of the great (original) time travel books is "Melmoth the Wanderer" -- not exactly time travel, but the hero/villain is cursed to be immortal, and wanders through time. The structure of the book is like Russian dolls. At one point in the book the reader is seven stories deep in the plot, and has to come all the way back out again. The book was written in the 1800's.

Peter T.