The reference books for PMP other than PMBOK Guide are mentioned below.
Effective Project Management: Traditional, Agile, Extreme, Hybrid | Author: Robert K. Wysocki | Publisher: Wiley
Essential Scrum: A Practical Guide to the Most Popular Agile Process | Author: Kenneth S. Rubin | Publisher: Addison-Wesley
Project Management: A Systems Approach to Planning, Scheduling, and Controlling | Author: Harold Kerzner | Publisher: Wiley
Project Managers Portable Handbook, 3rd Edition | Author: David Cleland and Lewis Ireland | Publisher: McGraw-Hill
Project Management: The Managerial Process | Author: Erik Larson | Publisher: McGraw-Hill
Fundamentals of Technology Project Management, 2nd Edition | Author: Colleen Garton with Erika McCulloch | Publisher: MC Press
Information Technology Project Management, 7th Edition | Author: Kathy Schwalbe | Publisher: Cengage Learning
The Project Management Tool Kit: 100 Tips and Techniques for Getting the Job Done Right | Author: Tom Kendrick | Publisher: AMACOM
Agile Practice Guide | Author: Project Management Institute | Publisher: Project Management Institute