Not a good book. After Programming in Lua, I'm very disappointed. The quality of these 'gems' is generally very poor. A few were written by obvious beginners, providing a worse introduction than PiL. Generally the editing was rubbish. The editors obviously didn't make an effort to bring the contributions up to scratch, much English was mangled, and there are even literal FIXMEs in the text.
There are two or three actually good gems, written by people who have obviously built up expertise and are willing to share their experiences with the language. Mostly, though, they are limited in what they can write by space, so that the rest can be given over to dross. Generally, the rest focuses on either the language (adding nothing to PiL) or computer science (rehashing well-known algorithms). Perhaps the lesson to learn is that Lua is such a simple language that the step from beginner to near-expert is simply to read Programming in Lua!