Ronald L. Graham, Donald E. Knuth, Oren Patashnik, Concrete Mathematics (2nd Edition).

• Comprehensive introduction to permutations, combinations, and their applications.

Richard P. Stanley, Enumerative Combinatorics.

• In-depth exploration of combinatorial mathematics, including permutations and combinations.

Wikipedia: Permutation

• https://en.wikipedia.org/wiki/Permutation
• Theory, formulas, and examples of permutations.

Wikipedia: Combination

• https://en.wikipedia.org/wiki/Combination
• Theory and examples of combinations.

NPTEL: Combinatorics

• https://nptel.ac.in/courses/111/106/111106112/
• Video lectures and notes on permutations and combinations.