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.

Full 3 volumes of Free pdfs on IA:

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

I used to love browsing back issues of journals on school/college Mathematics by volume using /scimag path of old libgen. Journals such as

American Mathematical Monthly

Mathematics Gazette

College Mathematics

Mathematical Intelligencer

and so on

The interface let you navigate to a journal of choice, say Nature then choose an year, volume and number. Then you could read the articles in that issue.

Scimag is no longer supported on the new libgens. However, one could make do using their Series search feature. This feature is still supported in name, however no article shows up.

If anyone knows of a work around would love to know.

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

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