The Complete Guide to Western Horsemanship by J P Forget, its quite an old book but you may be able to find a copy.
I would also suggest you have a look at the following website www.westernscene-uk.com as they usually have some books and also on the www.aqha.com website too for videos too.
Enjoy.
The western horseman books are good... esp craig camerons book ride smart ..lots of basic to more advanced infomation and how too and great pictures....