Pro Drupal Development (by John K. VanDyk and Matt Westgate, published by Apress; get it here) is a great little book. I know a bit about Drupal, and have written a couple of modules, but always felt like I was skirting the edge of some dark lake I dared not step into; when I did get in, I was quickly overwhelmed by the currents, got wet, and struggled out as soon as I had what I needed. (OK, a bit melodramatic, but you get the picture.) My knowledge was mostly gleaned from the handbooks on the Drupal site, which vary widely in quality, some being excellent and complete, others patchy and inaccurate, or for obsolete Drupal versions. I've also dug around in Drupal code a lot, but a clear understanding of the architecture continued to elude me.
This book, however, shines a clear strong light into Drupal's innards. I feel like it's written for someone like me: pretty technical, fairly able to make sensible inferences if given decent examples, and with some experience of Drupal of a user and dabbler. The chapters are pretty terse, but pack in some excellent code examples and fragments to help with common tasks. For example, I've been working a bit on my Last.fm module this week, and this book helped me to:
And probably other stuff I've forgotten. Things I had been working out from the handbooks, by making inferences from other people's code, reading forum posts etc., were covered briefly, pleasantly and clearly. They've probably saved me a good few weeks of scrabbling around. The book covers most Drupal concepts with enough depth for you to get a good overview of the whole architecture, as well as giving you practical snippets (table of contents here).
Highly recommended, and a snip at $22.50 for the ebook. No Drupal developer should be without it! Especially useful if you're not a complete Drupal nut but want to be able to write modules properly.