Draft of Enumerative Combinatorics - volume I

Richard P Stanley has the draft of the second edition available at http://www-math.mit.edu/~rstan/ec/ec1/. The book is a classic and highly recommended if you are interested in combinatorics. The book assumes advanced knowledge of mathematics (commutative integral domains, generating functions, etc). I am reading the first chapter and I've been ignoring some of the rigor to get the most from it.