The current “widely-supported” P4 Specification is P4_14 (version 1.0.3) Click here for this “widely-supported” P4_14 Specification.
To get answers to some of the frequently asked questions on P4 check out the P4 FAQ
The Original P4 Paper
Pat Bosshart, Dan Daly, Glen Gibb, Martin Izzard, Nick McKeown, Jennifer Rexford, Cole Schlesinger, Dan Talayco, Amin Vahdat, George Varghese, David Walker ACM Sigcomm Computer Communications Review (CCR). Volume 44, Issue #3 (July 2014)