What are some of the best mathematics books?

This comes to mind:

"Chicago undergraduate mathematics bibliography"

-> https://www.ocf.berkeley.edu/~abhishek/chicmath.htm

Prior discussion of the above link are as follows:

https://hn.algolia.com/?q=https%3A%2F%2Fwww.ocf.berkeley.edu...

Concrete Mathematics

Could you mention what level you are working at to get level appropriate responses?