PMBOK Guide is a must read before taking up the PMP exam. Even if you read Rita Mulcahy book or Headfirst, both those books would themselves suggest going over the PMBOK Guide atleast once.
But there are two other documents that are an absolute must in addition to the PMBOK Guide and both are PMI owned.
1. PMP Exam Content Outline – In addition to the percentage of items on the test Process Groups wise, the most important topic in this standard is the Role Delineation Study. It is not a static document and is updated timely. The current one is linked here – http://www.pmi.org/-/media/pmi/documents/public/pdf/certifications/project-management-professional-exam-outline.pdf
2. Code of Ethics and Professional Conduct – There are certain exam questions that deal with PMI’s code of ethics. This is again another document that you should read before the exam. The document is linked here – http://www.pmi.org/-/media/pmi/documents/public/pdf/governance/code-of-ethics-and-professional-conduct.pdf
NOTE – Please refer to the PMI website for the latest version of these documents