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.

A good book is  Discrete mathematics by N. Biggs. His writing style is very clear, it offers many good exercises (with solutions online) and covers interesting topics.
As for exercises I am in the process of creation of a page with exercises from various fields of mathematics. Check http://exwiki.org . The list is currently small, but you can    post useful exercises that you will encounter along the way when studying combinatorics.
