I am a beginner in this craft. Do you know where can I download a good book about basics of discrete mathematics?
Edward A. Bender & S. Gill Williamson, Foundations of Combinatorics with Applications, is available in both formats here.
Both have a strong emphasis on combinatorics, and both do a significant amount of graph theory. The Bender & Williamson is a bit more computer-science oriented and covers in greater depth some of the topics that they have in common; the Lovász & Vesztergombi covers some elementary number theory.