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.

Apart from the book suggestions given here, you may also like to take a look at MIT OCW's Combinatorics: The Fine Art of Counting. 
Although designed for High school students, few problems that might make you think. 
