Figure 1: Open GL ES 3.0
I found this book to be a very good introduction to opengl es. It has a nice progression through the chapters building up to more advanced topics near the end, for example terrain generation and advanced shader effects.
This book seems to be aimed at users new to opengl es or users transitionaing from other versions of opengl to es 3.0. If you're familiar with other versions of opengl you will likely use this book as more of a reference to look up the differences in the function calls.
I was very impressed by the fact that the example code can be compiled on multiple platforms, including dekstop and mobile. The example code uses various helper functions which get created and explained as you progress through the book. All code will run on linux, windows, mac, andriod and ios devices with build instruction for each platform at the end of the book.
My main crtisism of the book would be that it explains some of the maths, but not in enough detail to be useful for someone who wants to understaanding the maths in depth. Therefore if you want to learn exactly how some of the math works another book will likely help however I don't think in depth understanding is essential early on.
Most of my knowledge regarding opengl has been gained through lots of googleing, this book certainly helped me fill in blanks and explained things in much more depth than trawling through the internet.