A free sample chapter is available for download from the official Apress web page for the book.
First Edition, covering Drupal 5 (Chapter 8 - The Theme System)
The first chapter of the first edition was published on Dr. Dobb's (by special agreement with Apress) in April 2007 and is available online.
Second Edition, covering Drupal 6 (Chapter 1 - How Drupal Works)
The first chapter of the second edition was published on OStatic (by special agreement with Apress) in September 2008 and is available online.
The full book is not legitimately available online for free.