The Time Traveler's Wife, by Audrey Niffenegger (pub. 2003)
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So, this came out in 2003; I read it in 2014-15. In 2009, there was a movie adaptation, which I didn't see until 2022. Also in 2022, there was a TV adaptation, which I don't know if I'll ever get to see. The book is predominantly a romance, but since I generally don't read romance novels, I tend to think of it as science fiction. Still, it's... more contemporary than most SF, more grounded in reality (in spite of the very unreal premise). I did enjoy both aspects of the story, the romance and the time travel, but... it's hard to really explain... I liked the romance more, but it's so intrinsically tied up with the SF element that... well, you can't really think of one without the other. Anyway. There are two protagonists, and the story alternates between their perspectives, and their own distinctive voices.
Clare Abshire first meets Henry DeTamble in 1977, when she's six years old. At the time, Henry is 36, but that's because he's from the future. In real time, he's eight years older than she. He'll appear in her present (his past) periodically throughout the next twelve years, always coming from different points in the future. But the first time he meets her is in 1991, when he's 28 and she's 20 (at which point it had been two years since the last time Future Henry had visited her in the past). Once they meet in real time, they can finally start dating openly, and have something vaguely resembling a normal relationship. But it's still complicated by the fact that Henry frequently disappears to other times and places, and Clare must wait for him to return. It's also... well, it's always been a bizarre relationship. Obviously, they're not romantically involved when Clare is a child, but Henry knew that they would be, someday. Because he was coming from a time when they were already married.
Young Clare often begs Henry to tell her things about their future together, but for the most part he refuses. (This is the type of time travel story where the past cannot be changed, even with foreknowledge, but it can still be problematic to know too much in advance.) One might say that Future Henry plays too much of a role in determining Clare's destiny, even though he does withhold a great deal of information. However, the tables are turned when they meet in real time, because by then Clare knows all about him, even though he's still never met her. So from that point on, she's usually the one withholding information about the future from him. (All this kind of reminded me of the relationship between the Doctor and River Song on Doctor Who. Which fact is itself sort of a case of cause and effect getting temporally scrambled, because the storyline on that show was inspired by this book, though I saw the show before I read the book. Which kind of makes me a time traveler, I guess.)
Anyway. It's all very complicated, and I don't want to reveal too much of the plot. I suppose I should mention that Henry has a genetic condition which will eventually be called Chrono Impairment. He can't control when he disappears from the present, nor when or where he appears (in the past or future). He's always naked when he time travels, since nothing that's not a part of him, including clothes, can go with hm. So that's problematic in numerous ways. Eventually, he meets a doctor named David Kendrick, who does research on Henry's condition. And... there are numerous other characters throughout the book, including Henry's widower father, Clare's family, a woman named Kimy who is a friend of Henry's family, and several other friends of Henry and/or Clare. Their two best friends are a couple named Gomez and Charisse. And... well, lots of things happen. Good things, tragic things, funny things, annoying things, romantic things... all kinds of things. And I don't want to spoil any of it.
So, I'll just say... I really liked the story, and the writing. I liked Henry and Clare (most of the time). I liked how confusing it all was, and how much sense it all made. I liked all the feels the book gave me (both happy and sad). I didn't so much like the sex scenes, because I'm a major prude, but at least they were relatively brief. And... that's all I can think to say.