It's interesting to watch the conspiracy theories over at Amazon.com. Perhaps the publisher and the author are sneakily in cahoots to sell books about Drupal 5 to unsuspecting Drupal 6 users! As is often the case, the truth is much less interesting. In the flurry of chapter revisions, final proofs and corrections, the detail of putting "Covers Drupal 5" on the cover was simply overlooked. I'm willing to bet the second edition will say "Covers Drupal 6" on the cover.