An excellent and highly readable introduction to Processing (and programming in general). Shiffman does an excellent job of introducing concepts in an intelligent order and incrementally building on them and moving on to more interesting and complicated matter. After completing the book I can confidently say I have a solid base of understand of not only the core of processing but how, where, and when I should look for the information, techniques, and additional libraries and topics to continue developing projects beyond the scope of this introductory text. This is more of a project based, practical application, learn by doing textbook than the (also excellent) Reas/Fry text which I would suggest as a follow up as it seems to dive a little deeper into certain programming subjects and artistic theories, but without the project based (and classroom tested) approach that shiffman takes.
Now on to read The Nature of Code!