I started with Petri Nets back in the mid-80's with the book "Petri Net Theory and the Modeling of Systems" by J. L. Peterson (highly recommended, as I've seen more recent books, but they tend to be more academic than practical [not that there's anything wrong with that]) and have actually seen it recently in a technical bookstore. Having now seen what Google finds on the subject, I would agree with it and say that http://www.daimi.au.dk/PetriNets/ is a great starting point. I had it bookmarked from long ago and see that Petri Nets are still smokin' ;-)