Is there an introductory book on programming for the BeOS (something that starts with "Hello World" and goes on from there)?

Currently the first, best place to get technical information about programming the BeOS is the Be Developer Library:

http://www.be.com/developers/developer_library/

The Be Book contains architectural information that should help get you started:

http://www.be.com/documentation/be_book/index.html

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:

http://www.be.com/developers/developer_library/tutorials/approachingBe.html

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:

http://www.be.com/developers/developer_library/tutorials/index.html

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:

http://www.be.com/purchase/bookstore.html