Awesome Textbooks 
A curated list of awesome, free, and/or open source textbooks.
I've noticed a fair amount of open source texts for things like programming languages, but math and physics textbooks tend to be less accessible. This is a place to collect those that are out there. If you'd like to contribute to this list, please make a pull request.
Contents
Chemistry
Electrical Engineering
Mathematics
Physics
- Photonic Crystals: Molding the Flow of Light (John D. Joannopoulos, Steven G. Johnson, Joshua N. Winn, and Robert D. Mead, 2008)
- Physics of Light and Optics (Justin Peatross and Michael Ware, 2015)
- Understanding the Finite-Difference Time-Domain Method (John Schneider, 2010)