Because of phpBB's popularity, there have been books published to assist users in their use of it. Check them out:

phpBB: A User Guide (Packt, February 2006)
Authored by members of the phpBBHacks.com Support Team and Community!

Building Online Communities with Drupal, phpBB, and WordPress (Apress, December 2005)
Co-authored by a former member of the phpBBHacks.com Support Team!

Internet Intern: phpBB 2 (Data Becker, October 2005)
Co-authored by members of the phpBBHacks.com Community!

Building Online Communities with phpBB (Packt, May 2005)
Authored by members of the phpBBHacks.com Support Team and Community!