The web site has a list of books. I've read parts of Ben Fry's book, and highly recommend it. It is more concerned with visualization techniques than with other aspects of Processing, and is not a general introduction to Processing. I have several books by Andrew Glassner in my library. Though I haven't read his Processing book, he is an excellent author of technical books. Daniel Shiffman's book should also be very good, judging by the source code and documentation on his web site. A new Kickstarter-funded book by Shiffman, The Nature of Code, may already be out.

The following is a short list of books on computational methods and computational art that I will build as the course progresses.

Computational Art

Computation and Culture

Computational Methods and Programming

History of Mathematics