Short answer: One. But, you better read the next few paragraphs.
The NCEES passed a vote in 2011 to go to a closed-book exam for PS licensure. The NCEES is avid about protecting its questions. Because sample questions must be inserted into each year's exams to calibrate them for future years, the Council must ensure questions are not jotted down in reference books and taken out of the testing center.
When I took my PE exam in 2009, PS examinees were wheeling in small libraries of books (not recommended, btw). Most of them had large collections of voluminous tomes.Basically, this is really a blessing for PS testers. No more worry about having the best references, purchasing many references, and wheeling them into your testing center from the parking lot.
How many books are allowed in the Principles and Practice exam? Just one, and they give it to you in the exam.
NCEES sells a set of 50 questions to prepare you. They also offer a download of the provided reference book for examinees.
Breakdown of subjects on the exam: PS Exam Specifications
Free download of the PS Supplied Reference Handbook. Note of caution: check back for latest version as your study progresses. NCEES says they may update the reference book periodically.
If the 24 page booklet makes you think you will really need to know your stuff, I'm with you on that. 4 hours for 67 questions, and then 2 hours for 33 more... and just 2 dozen pages of support. Better study up!
Here are some more options to study and prepare:
1,100 Solved surveying problems (One to keep on the shelf). The ppi2pass promo code should work for this product as well.