The best way to overcome your fears is to attack them head on! If you're just starting off, using a pre written engine may make things easier, but eventually down the line, you can't always rely on someone else's code. That's one lesson I've learned in life. I couldn't avoid writing my own code forever.
Tbh, OpenGL ES isn't really tough once you know how to use it. And if you think OpenGL ES is bad, imagine living in 1997 and using Direct3D 3.0, now THAT was hard (but I'd give ANYTHING to go back to those days)! It's important to focus more on having a working engine first, then optimize it as necessary.
As far as books go, I have only a few books specific to writing game engines, but they're DirectX based. If you know OpenGL ES well enough, porting it won't be too hard though.
1. Game Coding Complete by Mike McShaffry
2. Tricks of the [Windows] Game Programming Gurus by Andre Lamothe
3. Programming a Multiplayer FPS in DirectX by Vaughan Young
All really good books (#2 is a bit old though), and the underlying principles of game engines aren't specific to DirectX either. That's why I recommend them. Unfortunately, I don't have any game engine books that use OpenGL, nor do I have any books on OpenGL ES.
Oh, I forgot to ask you this, how much experience with C++ and OpenGL ES do you have?
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