If you want to get at all serious about taking Drupal beyond a simple installation, Pro Drupal Development is going to be a massive help to you. If you’re anything like me you’ll come back to it again and again for reference and to try out some new ideas.
It covers all kinds of development areas such as building your own modules, the form API, themes, and even best practices. All this kind of stuff can be found online, but it’s a big help to have it all clearly explained in one place. Sometimes you just need someone to guide you through the maze of options that Drupal offers.
Much of the book isn’t for the faint-hearted. You need a good working knowledge of PHP and some MySQL. Most books start you off nice and easy, but here the early chapters are very much a leap into the deep-end. There’s a certain amount of reliance on just accepting fairly esoteric Drupal stuff until it’s explained more clearly later in the book.
The book also comes with plenty of examples, which you can download for free from Apress. These really help you get your head around some of the trickier ideas (I found one or two of the examples didn’t work as described – but to be fair there are some errata by the author on the Apress site).
In summary – this book is an excellent buy if you want to take Drupal to the next level but don’t know where to start.