Finding The Right Book
The best place to start is with a copy of the Candidate Information Bulletin. This contains the examination content outline and reference material used to create the examinations. It also provides a list of reference material allowed in the examination center (if your examination is open book).
All Candidate Information Bulletins (CIB) are available from http://psiexams.com.
Once you have the CIB you can find the books by either using the Search by State Exam category to find all books useful for your examination or use the trade categories to search. Finally, you can search by the publisher.
PSI only sells new books at the same price as the publisher.