Where are good resources to study combinatorics? I am an undergraduate wiht basic knowledge of combinatorics, but I want to obtain sound knowledge of this topic. Where can I find good resources/questions to practice on this topic?
I need more than basic things like the direct question 'choosing r balls among n' etc.; I need questions that make you think and challenge you a bit.

Lots of good suggestions here.  Another freely available source is Combinatorics Through Guided Discovery.  It starts out very elementary, but also contains some interesting problems.  And the book is laid out as almost entirely problem-based, so it useful for self study.
