PMP Certification books
Among the many books available in the market for PMP, preparepm has reviews for the best ones here.PMP Exam Textbook 4.0. The textbook is a great free resource for anyone planning to take the exam before July 31, 2013.
A Guide to the Project Management Body of Knowledge (PMBOK Guide) -- 2008 Edition
PMBOK is the main resource on which the exam is based. When you become a member of PMI, you will be able to access the PMBOK online on PMIs site. If you prefer to read from a book rather than your monitor, then you may have to buy it. Most people find the PMBOK a little boring to read though. The fourth edition of PMBOK was launched in Dec 2008. From July 1st 2009 onwards, the exam is based on the fourth edition.
Other Excellent PMP certification books
The PMP Exam : How to Pass On Your First Try by Andy Crowe is another very good book. The book covers the key concepts in clear language. If you find the PMBOK too dense and difficult to understand, then this book can suffice as your primary preparation material for the exam.
Head First PMP: A Brain-Friendly Guide to Passing the Project Management Professional Exam by Andrew Stellman and Jennifer Greene presents Project Management concepts in a light and interesting fashion.
PMP: Project Management Professional Study Guide by Kim Heldman is an extremely good book. The book is very comprehensive and has good practice questions.
PMP Project Management Professional Study Guide (Certification Press) by Joseph Philips. The book covers all the things you need for the exam. It is very comprehensive, and one does not even need to read the PMBOK much after reading this book.