Just my opinion but i think the Pratt books are some of the worst text books i've ever read , and i've read lots.
Looking at the ppl confuser and trying to find the answer to your question in the Pratt books is a nightmare . I've just bought the Trevor Thom ones and stuck the Pratt things on ebay.
I must point out i come from the school of pass the exam rather than understanding why you passed in the first place.