I bought the WebGL Programming Guide after previously reading the excellent OpenGL Programming Guide (the iconic "Red Book") from the same series. The WebGL Programming Guide is a good introduction to WebGL, covering many fundamental 3D graphics topics. There are a few notable topics missing though, including calculation of surface normals using the cross product (although this is briefly mentioned in a footnote in chapter ten), specular reflection and cube map textures.
As other reviewers have commented, the authors have chosen to abstract out utility functions and matrix operations into helper libraries in the sample programs. Whilst I understand the reasons for this, I think the explanations of these libraries could have been more comprehensive. For example, the utility function for getting the WebGL context calls other functions in the Google webgl-utils library and Khronos webgl-debug library (the only reason that these libraries are imported by the sample programs), but this isn't explained in the book. In addition, the utility function for initialising shaders is not explained until chapter nine, despite being used in every example program in the book from chapter two onwards. The book also treats the methods in the matrix library inconsistently, both in terms of the level of mathematical explanation, and the formatting of method descriptions in different chapters.
Despite the minor criticisms above, I would still give the WebGL Programming Guide four stars. It covers everything needed to start developing interactive 3D graphics in WebGL.