Looking for resources to better learn pigeonhole principle and other combinatorical arguments

Question

While I claim to understand the premise of the pigeonhole principle (if $n > k$ then inserting $n$ elements into $k$ boxes results some boxes containing at least two items), I am still quite bad at applying to principle in practice. Therefore I am looking for the resources (books/sites/psets/old course material etc.) to better learn the pigeonhole principle and other combinatorical arguments. The material should preferably contain problems with varying level of difficulty and expose me to the "most common" types of situation in undergrad/grad level in which a combinatorical argument can be used.