Herbert Wilf has put several of his books online for download. I particularly recommend generatingfunctionology, which is an excellent account of the uses of generating functions in combinatorics. I regarded generating functions as a sleazy trick before I read that book. A=B, a book he co-wrote with Petrovsek and Zeilberger on combinatorial sums, is also very interesting. It turns out there is an eminently-implementable algorithm that will show, for a large class of formulas, when two such sums are equal.