You could try the following books:

Programming PIC Microcontrollers With PicBasic by: by Chuck Hellebuyck
Experimenting with the PicBasic Pro Compiler by: Les Johnson

I have read about them, but I haven't read the books themselves.
(some people may say: it would have been better I had read the books)

Another way of learnig is once you have got a bit familiar with PBP and the PIC Hardware:
look at the many code examples available and try to understand what they are doing (or are supposed to do) rather than just copying them into into your project.

Adittional recommendations:

Try MicroCode Studio http://www.mecanique.co.uk
This is really a great Product and the "Plus" version is worth the money.
The ICD Feature saves a lot of time when debugging.

For small projects try the 16F628 (or even smaller: 12F675)
The 16F628 has more features than the 16F84 and is even cheaper!
And it does not need external parts like a crystal.

If you need more I/O Pins go for the 16F868 / 16F877.

Take care with the 18F series, they appear to be a bit "tricky"
(I'm currently struggling to migrate one of my projects from 16F876 to 18F252.)