O'Reilly's Programmer's Guide to Drupal, written by Jennifer Hodgdon is a solid book for Drupal developers of all skill levels. I'd argue that it is one of the better books for PHP developers wanting to learn more about Drupal. It provides a wealth of solid information on a nice array of topics that professional Drupal developers should know. It's not a long read (less than 100 pages of actual content), but the structure and variety of topics covered makes it a great reference for best practices and intermediate to advanced "what's the best way to do this?" topics in Drupal development.

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