Just a few books I've found useful over the years in different areas.
Optics:
"Optics" - Hecht
It's not cheap but it provides a very solid foundation in optics, both basic and relatively advanced. It's an essential read for anyone wanting to study optics in a more advanced manner.
Applied Maths:
"Advanced Engineering Mathematics" - Kreysik
This book covers ALOT of topics, and provides examples for each. It doesn't give you proofs, but it is very useful as a reference guide to many mathematical tools and their usage.
Mechanics:
"Quantum Mechanics" - Bransden & Jocaine
Comprehensive, if a little dense in parts, useful for a background in more advanced quantum areas.
"Classical Mechanics" - Chow
A solid mechanics primer, it covers most of the important areas, and sets one up for tackeling more advanced topics down the line. Definitely a necessary read for anyone looking at taking Classical Mechanics as a full theoretical subject at third level.
Hope the above can be of use to someone