The PMBOK is the source for all the material used in the PMI examination. However, it is not organized as a study guide or narrative text, but rather as a reference guide. It is difficult to master the material in the PMP exam by only studying the PMBOK.
A number of study guides are available, as both online sites and texts, which are available in most technical bookstores. These guides reformat the material in the PMBOK into logical blocks that allow you to understand and memorize the information. The guides are also helpful in pointing out the common methodologies behind the PMBOK methodology.
A number of companies also offer formal training and review classes for the PMI exam. These classes often provide additional study materials not available elsewhere.
Reviewing sample test questions is the best way to prepare for the PMI exam. PMP exam questions are often misleading, confusing and focused on minutiae of the PMBOK.
Reviewing as many exam questions as you can find will assist you in identifying common mistakes made in interpreting questions and understanding how the hierarchy of information in the PMBOK is applied.
You can find sample exam questions in most study guides and from online resources.
While the PMI requires proof of experience as a Project Manager in order to be certified, your accrued knowledge may not prepare you for the PMP examination. The test is drawn from the specific methodology and nomenclature of the PMBOK, so unless your previous experience was in a PMBOK environment, it will not prepare you for the exam.