My very first was Naive Set Theory by Paul Halmos. Way over my head in 7th(?) grade but my first intro to math beyond pre-algebra stuff.

Lately I've enjoyed, but did not finish, the Joy of Abstraction by Eugenia Cheng, on category theory. And there was a differential geometry book whose name I have forgotten but whose exercises I really enjoyed, because I could do them in my head while riding the bus, just by thinking about them.

I'm not particularly well read on mathematics (had a lot of math in college, hardly any since) but I would like to circle back to reading more at some point.