recommended
Segal, M., & Akeley, K. (1994). The Design of the OpenGL graphics interface. In Silicon Graphics Computer Systems.
Marshner & Shirley. (2016). Using Graphics Hardware, section 17.1-17.10. Fundamentals of Computer Graphics, AK Peters. (ncsu). Note this describes OpenGL, but is quite relevant to webGL.
optional
Mark, W. R., Glanville, R. S., Akeley, K., & Kilgard, M. J. (2003, July). Cg: A system for programming graphics hardware in a C-like language. In ACM Transactions on Graphics (TOG) (Vol. 22, No. 3, pp. 896-907). ACM. (ncsu).
Parisi, T. (2012). An introduction to WebGL. Chapter 1 in WebGL: up and running. O'Reilly Media.
Angel & Shreinger. (2014). An introduction to WebGL programming. A video in the SIGGRAPH university series. It's long, watch at least until 37:30, when GLSL is introduced.
online at our library
Cozzi's WebGL Insights, 2016
Cantor's WebGL Beginner's Guide, 2012
Parisi's WebGL: Up and Running, 2012
elsewhere
Matsuda & Lea's WebGL Programming Guide (not free)
Eck's Introduction to CG (online and free)
Angel & Shreiners Interactive Computer Graphics with WebGL (author site) (pub site)
Eric Haines at Udacity
Angel & Shreiner's SIGGRAPH 2014 Tutorial
This course's video list
Angel & Shreiner's SIGGRAPH 2014 tutorial
WebGL Inspector (Chrome Extension)
Giles Thomas' Learning WebGL lessons
Mozilla's tutorial
Khronos' tutorial
WebGL fundamentals
Wayne Brown's lessons
WebGL reference card
WebGL Programming Guide
Real Time Rendering's WebGL page
Morgan McGuire's js intro for graphics programmers
Powell's intro
Kode80's intro
Wayne Brown's shader intro
Florian Boesch's WebGLStats
webgl
threejs