My Favourite: generatingfunctionology by Herbert Wilf.

The exposition is crystal clear, and it's full of insight and interesting techniques. It shows how you can use generating functions for a number of things, for example: evaluating sums, obtaining asymptotic formulas for strange sequences.
I think that if you bought Concrete Mathematics, you will quite likely enjoy this book very much. - Ah!, and it smells good too ! -

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