I’m not sure that there’s an additional book that explains the NAP more throughly, but I could be mistaken. It may be a good idea for you to pick up a basic book on Hermeticism and/or basic ritual practice, which could help you to understand the very basics.
If you’re coming at this brand new, I’d maybe suggest you read the book through, from cover to cover, 2/3 times…then absolutely and unconditionally believe that it works (like you just know that the sun will come up in the morning). If you’d like a deeper understanding then take a look at a couple of the links below …
https://thefoolspeaks.com/showthread.php?t=609
Again, patience…practice…belief, and good luck with accomplishing your goals