[mohamez]

Any good book about the history of mathematics that will teach you a natural historical development of concepts to reach more generalizations.

History of mathematics will give you a very subtle entry into the minds of mathematicians and the motivation behind their theorems.

This will surely make you more appreciative of subjects and concepts you are learning.

[mrazomor]

The Mathematical Experience -- https://www.goodreads.com/book/show/1113522.The_Mathematical...

It's touching many areas. For some it explains how they were developed or the controversy around them (e.g. the definition and use of infinity).

[mharig]

Foundations and Fundamental Concepts of Mathematics, by Howard Eves