There are many resources, and ongoing and forthcoming activities on Petri Nets. The Petri Nets World (these Web pages) assembles information about such activities under supervision by the Petri Nets Steering Committee.
Moreover there is an excellent article about Carl Adam Petri, his exceptional life, and work. The article Carl Adam Petri and "Petri Nets" was written by Wilfried Brauer and Wolfgang Reisig and was translated from Informatik-Spektrum, Vol. 29, Nr. 5, pp. 369-374, Springer Verlag, 2006. You can download it here as [pdf] or [doc].