A set of documents you may find useful:

• Getting Started Guide
• VM Setup Tips
• Running p4app on Mac OS X
• PWOSPF Specification
• Debugging with p4app

Learning P4:

• P4 Tutorial 1 - Introduction to Data Plane Programming
• P4 Tutorial 2 - Basics Part I
• P4 Tutorial 3 - Basics Part II
• P4 Tutorial 4 - Basic Tunneling Application