Seconding the recommendations of Heinlein (especially The Door Into Summer) and Gerrold’s The Man Who Folded Himself – which was written specifically to contrdict Heinlein’s deterministic theory of time. Also intriguing, though dated: H. Beam Piper’s “Paratime” stories, including Lord Kalvan of Otherwhen.
Not everybody’s cup of tea, but something that was strongly recommended to me by a gay friend, a fellow SF fan who knew I had no issues with gay people and gay-oriented fiction, and which I found fascinating reading. Do you want to try a bit of online fiction that – well, know what a “Mary Sue” is? The author’s idealization of herself, or himself, injected as a character. This is one of them, but just as some genre fiction works transcend the limitations of their genre, this one rises above the horrendousness of the typical Mary Sue story by the author’s ruthless self-honesty in the sequelae.
The story is Dan Kirk’s Do Over series. The set-up: Down-on-his-luck 35-year-old gay ex-Navy veteran jumps at the chance to make $10,000 by participating in a scientific experiment – which turns out to be, they’ll inject him with some (unspecified) chemicals and scan his brain with a MRI machine – and then transmit his consciousness back in time where he can observe his own past for 20 minutes. Only he discovers he’s stuck there, 35-year-old man with memories up through 2004, in his 12-year-old body in 1981. And far from being an observer, he can act – and discovers he can make some changes
Book One, “Do Over”, is the “Mary Sue” story – but is quite well done. Book Two, “Do Over Redux,” posits he needs to repeat the process – and things go disastrously wrong. Well, if at first you don’t succeed… And “Do Over Redux” ends with what I think is the most inevitable, satisfying conclusion I’ve ever read.
Book Three, “Doing It Right”, is fluff by comparison, but fun. And it makes the important point that even in an idealized world, there are sometimes tough – and costly – moral choices to make. And Book Four, “Let’s Do It,” examines the same general plotline-comple from a different perspective.
Like I said, it’s not everyone’s cup of tea – but I found it a refreshing variation on the concept of time travel and alternate universes.