A new book, OpenGL Insights, that I helped put together is about to come out. We put some good stuff on the website, including an OpenGL Pipeline Map, which includes ES 2.0 for those of us doing WebGL, and several sample chapters, including The ANGLE Project: Implementing OpenGL ES 2.0 on Direct3D by Daniel Koch and Nicolas Capens, which goes pretty deep under the hood of ANGLE.
Scott and I also wrote a chapter, WebGL for OpenGL developers, based on our experience in the early days of Cesium. The table of contents has WebGL-related chapters tagged.
Although I haven’t physically held a copy of the book yet, there are some reports of it arriving to London early.