I have a few different books on pranayama but this is by far the best. It goes into more detail about prana and deeper also into the practicalities of pranayama and the effects.
As always from the Bihar School, it is clearly written, well laid out with the first part being theory followed by the practical applications in the second half. Saying that, this is not a subject for the absolute beginner in yoga and as is clearly stated pranayama must be given huge respect and is not to be toyed with. It can give great benefits but abused it can cause great damage. I also think its very useful from a teaching point of view.