Elin Waring and Jennifer Marriot have teamed up to write "The Official Joomla Book."
Huh? Official? Yes, this book is official sanctioned by the Joomla project and the proceeds go to benefit the Open Source Matters foundation of the Joomla project.
I spent some time with book at my nearby Barnes and Noble and found it to be very well written with a lot of advanced tips as well as foundational material. It is written primarily for Joomla 1.5, but includes sections that explain the new features that are available in Joomla 1.6, so it has good "staying power" with both versions of Joomla.