Well, gee. Nuts. Yet another "wish I liked it better" book. Yet another "it's probably me, not the book" book. Because it won both Hugo and Nebula Awards for "best novella". (See the Amazon page for other encomia.) And (in theory, even better) it won a rave from Katherine Mangu-Ward on a Reason podcast back in 2019.
And I didn't care for it at all. Couldn't get interested in the characters or their stupid problems. Didn't like the obfuscatory "look. ma, I'm writing" prose.
But here is (as far as I understand it) the story: it's the story of two shape-shifting time-travelers, agents "Red" and "Blue", who are trying to alter the various time strands so their side winds up to dominate the eventual future. This involves a lot of gory slaughter. Red and Blue leave cleverly hidden notes to each other as their paths through spacetime cross. At first, they are taunting. Then admiring. And eventually, they fall head over heels in love.
They both use "she/her" pronouns for themselves, so it would be easy to call this yet another lesbian sci-fi story. (It would be my third recent one in a row; apparently it's a required theme to get attention of influential critics?) But that's problematic, because it's not clear that Red and Blue are even human. (I didn't get a handle on that.)
To repeat: you could very well like this book very much. But I think I'm going back to the old reliables for the foreseeable future: Heinlein, Herbert, Stephenson.