Having read several Books on Magento, I have to say that this is probably the best one for a developer. Most other seem to teach you how use the admin interface, but not how to develop custom module. This is the first book I've read that actully takes you through the code structure of Magento, step by step, and explains what's happening.
The only criticism I have is that there are quite a few mistakes throughout the book (typos, missing examples, etc) which can make it difficult to get the example code to work (I bought the first edition). But it's nothing that a little common sense can't solve!