The PMBOK® Guide, Sixth Edition (2017), is the international standard for project management from the Project Management Institute (PMI). This revision of the guide now includes sections in each knowledge area on Agile approaches and Iterative and Adaptive environments. It also contains more emphasis on strategic and business knowledge, as well as the essential skills for project management success. Traditionally, the guide represents the standard that all certified project managers (PMP®, CAPM® and PMI-ACP® credential holders), project offices and project managers should use going forward.
NOTE:  PMI Released the PMBOK® Guide, Seventh Edition in August 2021. The release of the PMBOK® Guide, Seventh Edition does not greatly impact the PMP exam content for the foreseeable future. Please refer to this article for relevant information:  PMBOK® Guide 7th Edition & PMP® Exam Changes