Discrete Mathematics lecture note written by P. Csikvári: pdf.
Discrete Mathematics lecture note written by P. Csikvári (in Hungarian): pdf.
Discrete Mathematics Problems written by P. Csikvári, Z. L. Nagy and D. Pálvölgyi (in Hungarian): pdf.
A. E. Brouwer and W. H. Haemers: Spectra of graphs, pdf.
J. Matousek: Thirty-three miniatures: mathematical and algorithmic applications of linear algebra, pdf.
R. Stanley: Topics in algebraic combinatorics, pdf.
Combinatorial nullstellensatz: Alon's paper pdf.
Generating functions: Wilf's book pdf and a short summary pdf.