I'm a graphics person, not a developer, but I picked up "Processing: A Programming Handbook for Visual Designers and Artists," and it's awesome - tons of visual examples and it walks you right through graphics from the beginning of the book. With your background I'm sure you'd skip some of the introductory stuff, like what the difference between float and int is, but even if you do skip that stuff, there's plenty of graphics/animation/typography info in the book to keep a graphics person satisfied. You might see if you can browse a TOC for the books you're looking into buying.

I've been meaning to review the available books for some time now.  Thank you for getting me to finally start writing.

There are three major books:  

Processing: A Programming Handbook for Visual Designers and Artists, by
Casey Reas and Ben Fry (R&F)

Learning Processing: A Beginner's Guide to Programming Images, Animation, and Interaction, by Daniel Shiffman (S)

Processing: Creative Coding and Computational Art, by Ira Greenberg (G)

Reas and Fry are the originators of the language; Shiffman and Greenberg both work with them on its ongoing development.  Speaking as an experienced C/C++ programmer with very little Java background, I've enjoyed all their books and learned something new from each of them.

One important thing to remember is that all of these books are aimed at a reader who is not only learning Processing, but learning programming for the first time, so a great deal of the text is review to an experienced programmer.  Another run through the fundamentals was useful to me.

R&F uses a very nonlinear arrangement of chapters, introducing elements of syntax, graphical concepts, I/O, math, and object orientation in an interleaved, as-needed fashion.  S and G, on the other hand, are laid out in a much more typical textbook style.

One outstanding feature of R&F is the visual table of contents.  It's priceless for a visually oriented reader trying to find the sketch that made that foggy texture, for example.  (p. 131)  S and G have much better indexes for looking up the details on Perlin noise.

Greenberg has some stylistic issues that got on my nerves, but I did get important mathematical and other details from him.

The most valuable learning resource for me has been the Teaching section of Shiffman's web site (http://www.shiffman.net/teaching), which contains summary and syllabus information for several of his courses.  I would dearly love to take his Nature of Code class on modelling natural processes and giving an organic feel to one's graphics.

In summary, I'd unhesitatingly recommend R&F to someone coming from an artistic background, or S for a reader wanting a more straightforward textbook approach, and G to fill in the gaps.  I've very happy to have all three.

I got 'learning processing', it's a brand new edition, so is very up to date.

Thanks all for your valuable input on this. So, overally, after also having a look at their websites and sample chapters/code, I am almost convinced to have a look at the Shiffman book first and see how far it'll get me. So far, browsing the book index, I am missing something like a quick-and-dirty tutorial not for someone with a visual/artistic background coming to Processing but rather something like this written for someone with a software-development background wanting to dive a little deeper into the visual things. Waiting for a "Processing Developers Notebook" [1]...

Cheers & thanks all,
Kristian


[1]http://oreilly.com/store/series/notebooks.csp