...
https://processing.org/reference/ : The reference page for Coding in Processing
http://learningprocessing.com/ A comprehensive book on the basics of processing by Daniel Schifman
The Nature of Code : A comprehensive free ebook more advanced book on visual coding from Daniel Schifman Schifman (free ebook)
https://www.openprocessing.org/: A community-based website for sharing creative coding examples
...