On Actually Using a P-TTL Flash
This article is concerned with comparing the available P-TTL options. For information on actually making the most of P-TTL, take a look at OK1000's introductory guide to P-TTL and (particularly if you are having exposure problems) the P-TTL flash guide from Pentax User UK.
Steve Jacob has produced an excellent series of slides called Flash Modes for Pentax. This includes tables showing the results of various combinations of camera and flash settings, and a section called "How PTTL works and what mode to use when".
More Pentax Flash Information
Jens Roesner has a page full of information about the Pentax flash system. Some of it is slightly dated, but even those portions are useful for historical perspective.