Here is a great book. I never read it but an author gives it much praise.

You can buy it.

But you can also get it for free.

In case your interested the book is about Generating Functions.
Which will greatly improve your Combinatorics skills and make you into a Show Off.

__________________
And he (Elisha) went up from thence unto Bethel: and as he was going up by the way, there came forth little children out of the city, and mocked him, and said unto him, "Go up, thou bald head"; "go up, thou bald head". And he turned back, and looked on them, and cursed them in the name of the Lord. And there came forth two she-bears out of the wood, and tore up forty and two children of them.
Second Kings 2: 23-24