Drupal 6 book recommendations

As we’ve just delivered a big lump of functionality onto our website using Drupal, in fact everything is now managed through Drupal except this blog, I thought that you might appreciate a heads up on the books we used during the development process. Getting hold of the right books early in your project will make things a lot easier. Whenever I’m taking on a new programming language or some other technology it always seems to take three books to really get a good handle on it.