You can find a lot of free e-books here.Books range from math classics to recently released ones gracefully violating copyrights.I simply believe knowledge should be free.

Some good books of my interest:

1.The art of Counting - Paul Erdos.

2.Combinatorics - Peter Cameron.

3.A course in combinatorics - van Lint and Wilson

4.Enumerative Combinatorics - R P Stanley

5.Research Problems in Discrete geometry - Peter Brass et al