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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.

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).

Foundations and Fundamental Concepts of Mathematics, by Howard Eves