Discrete Mathematics : Site
Wikipedia : Discrete mathematics
Donald Knuth : The Art of Computer Programming
Kenneth H. Rosen, Discrete Mathematics and Its Applications 5th ed. McGraw Hill
Richard Johnsonbaugh, Discrete Mathematics 6th ed. Macmillan
Norman L. Biggs, Discrete Mathematics 2nd ed. Oxford University Press
Neville Dean, Essence of Discrete Mathematics Prentice Hall.
Mathematics Archives, Discrete Mathematics