
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.
"Euler's Gem" by Dave Richeson
"A Companion to Analysis" by Tom Körner
"Elementary Number Theory: A Problem Oriented Approach" by Joe Roberts
It's Bressoud BTW, I endorse that too. Along with TW Körner „Companion to Analysis: A First Second and Second First Course” with Lang or Zorich as base. I wouldn't be as insistent on pencil at all times if it were to prevent broader reading or just expanded skimming.