For a single suggestion, "How to Think Like a Mathematician" by Kevin Houston.
A second suggestion: "A Companion to Analysis" by Tom Körner.
But it depends a lot on whether you want books about math, or books of math. It sounds like you want the latter ... at some point I'll get around to putting annotations on the choices in the list that would help distinguish.
https://www.topicsinmaths.co.uk/cgi-bin/sews.py?SuggestedRea...
For a single suggestion, "How to Think Like a Mathematician" by Kevin Houston.
A second suggestion: "A Companion to Analysis" by Tom Körner.
But it depends a lot on whether you want books about math, or books of math. It sounds like you want the latter ... at some point I'll get around to putting annotations on the choices in the list that would help distinguish.