Links

Miklós Bóna has a list of errata for the book (or a longer list for the first printing of the second edition) on his homepage for the textbook.

Wikipedia is, perhaps surprisingly, not a bad source for background material on basic definitions and results in many areas of mathematics, including combinatorics.

Some textbooks in combinatorics which are available online:

Reinhard Diestel's textbook on graph theory.
The 2nd edition (not the most recent) of Herbert Wilf's book "generatingfunctionology".