My formal introduction into Processing happened as a result
of signing up for a free class O’Reilly
was hosting on their website. The class taught how to use Processing for
displaying graphics animations, as well as using it to visualize data for the
Arduino microcontroller. After signing up for the class, I requested a review
copy of “Getting Started with Processing” from O’Reilly.
The book is thin and just under 200 pages. It is a good introduction to the Processing language, and is paced somewhat as an introduction to programming as well. Those who already program in other languages will have no problem, and those who haven't will probably be able to pick it up as well.
The examples in the book are all short and basic enough for just about anyone to follow. If you know C, C++, Java or any other OO language, you'll have no problem.
The last chapter describes how to read serial data from the Arduino and visualize that data with graphics. This is a fun chapter if you're new to microcontrollers and want to use something other than LEDs to monitor their output.
My only dislike about the book is the cost and that the information contained in it can pretty much be gleaned from Processing.org and from other books on Processing.
http://www.processing.org/learning/books/#fry
lists many books on Processing, but of a more advanced nature. Given that these books go well beyond the scope of "Getting Started with Processing", I would recommend looking at the other books first. If you are very new to programming in general, and want to minimize your investment, then this book is for you.
Overall, the book is a quick and straightforward introduction to data visualization, without the boring and complex math one would usually find.