1) https://eigen-space.org/discretebook/dbook.pdf
2) https://eclass.uoa.gr/modules/document/file.php/MATH727/252.%20Διακριτά%20Μαθηματικά/σημειώσεις%20Αθανασιάδη.pdf
3) Miklos Bona - A Walk Through Combinatorics
An Introduction to Enumeration, Graph Theory, and Selected Other Topics