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

• Comprehensive coverage of factorials, combinatorics, and their applications.

Peter J. Cameron, Combinatorics: Topics, Techniques, Algorithms.

• Explains permutations, combinations, and factorial growth in problem solving.

Wikipedia: Factorial

• https://en.wikipedia.org/wiki/Factorial
• Definitions, properties, and applications of factorials.

Paul Zeitz, The Art and Craft of Problem Solving.

• Problem-solving strategies and factorial-based puzzles.

NPTEL: Discrete Mathematics and Combinatorics

• https://nptel.ac.in/courses/111/106/111106086/
• Video lectures and notes on combinatorics and factorials.