What are the best quantum mechanics books available for someone looking to deepen their understanding of the subject?

Some of the best quantum mechanics books for deepening understanding include "Principles of Quantum Mechanics" by R. Shankar, "Quantum Mechanics: Concepts and Applications" by Nouredine Zettili, and "Introduction to Quantum Mechanics" by David J. Griffiths. These books provide comprehensive coverage of the subject and are highly recommended for advanced study.