Although almost everyone finds it surprisingly easy to start programming the BeOS, at the present time there are no third-party "Programming the BeOS" type books (we do know of a few currently being written).
Currently the first, best place to get technical information about programming the BeOS is the Be Developer Library:
The Be Book contains architectural information that should help get you started:
There is "Approaching Be", an introduction to the BeOS for the experienced C++ programmer who is just getting started with the BeOS. It offers insight into how BeOS programs are structured and how applications interact with the Servers and Kits of the BeOS. You can find this excellent starting point at:
Be is also developing an ongoing tutorial series for programmers interested in developing for the BeOS, and you can see this series on our Web site:
Finally, we have a listing of a variety of useful books recommended by our engineering staff for learning programming techniques which will be useful in programming on the BeOS: