If you are new to hearing what PMBOK is, It literally is Project Management Body of Knowledge.
From the book: The PMBOK® Guide is the standard for managing most projects most of the time across many types of industries. This standard describes the project management processes, tools, and techniques used to manage a project toward a successful outcome. This standard is unique to the project management field and has interrelationships to other project management disciplines such as program management and portfolio management.
There are lots of PM books that you could use since different organizations will have different methodology on how they practice Project Management. However for those who are taking the PMP certification, you know this is your a must have, a must read, a must know 🙂