I've been relearning trigonometry lately by myself for navigation and astronomy; not for work, just curiosity I guess. One book I've really enjoyed is Heavenly Mathematics by Van Bremmelen. It's a spherical trig textbook, but it's written by a math historian who describes how trigonometry was gradually developed over human history and he discusses its early proofs, methods and applications. I have to confess that the historical approach has really helped me develop a more complete mental picture and appreciation of the math itself. Understanding the "how" and "why" of its development, and seeing the early practical need and implementation for some of this stuff has made the topic a lot more engaging.

This is a great book if you already know good amount of Math. It helps you fit things into a bigger picture. Really appreciate the fact that something like this exists.

Full 3 volumes of Free pdfs on IA:

https://archive.org/details/MathematicsItsContentsMethodsAnd...

This is one of the best generalist books on mathematics ever published. I highly recommend it.

The only mathematics books I ever read was textbooks in school but now as adult I want to start from scratch.

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