In the interest of completeness we should also mention
PRC Tools.
For C programming PRC Tools works well enough. Segmenting your code (necessary for code larger than 32Kb) is a bit more cumbersome than CodeWarior, though.
For C++ programming, PRC Tools may not be suitable. The lack of support for exceptions and weak support for templates make it hard or impossible to adapt some C++ idoms to Palm OS development. CodeWarrior is definitely the better choice here.
Also, PNOlets (or ARMlets) are much easier to do in CodeWarrior. I'll spare you the details of why this is so.
The bad news is that both PRC Tools and CodeWarrior have not been updated for a long time and it doesn't look like they're being maintained either.
Finally, don't bother with the Eclipse-based PalmOS Developer Suite. It's primarily meant for Cobalt development.
The Lonnon Foster book is great, BTW. I have the previous edition (known as the "Palm OS Programming Bible") published by Wiley.