Starkeeper: I own and have found the Pop Piano Book a great book which teaches you how to play the piano in many different styles of pop piano. This book covers all the essential pop music genres:
http://www.harrisonmusic.com/book/pianobook.html

In addition to the book, I highly recommend getting either the audio CD or Midi file CD, so you can actually HEAR the samples of each pop style being played thus engraining it in your head (by ear) and then playing it on the piano. What the book does is explains the music theory behind what you're hearing to solidfy your knowledge.

If you're strictly an arranger keyboard player though, then this book may not be the book you want, as it focuses on teaching pop harmonies, bass lines, and emulating rhythmic grooves in a 'solo keyboard' playing style. Still, whether you're a solo pianist, play in a band, or on an arranger keyboard (w/auto accomp), the book (along with CD) will both improve & expand your pop piano playing abilities. I highly recommend the book. - Scott