Pro Drupal Development, Second Edition Apress
John K. VanDyk
Very good Drupal reference book
Using Drupal O'Reilly
Angela Byron, Addison Berry, Nathan Haug, Jeff Eaton, James Walker & Jeff Robbins
Excellent step-by-step tutorial book on using Drupal, also a good reference.
Building powerful and robust websites with Drupal 6 PACKT
Pretty good book on using Drupal.
Copies are available for in-office review in University Relations (contact DeWayne Purdy)
Other Drupal Training Resources
Lynda.uni.edu - Thousands of web design and development training videos, including Drupal 6 and 7.