Of the three books available on Processing at the minute, Ira Greenberg's book is the one for those with the least background in computer science. The author manages to explain some difficult concepts of programming using great concepts and simple English, and even includes an appendix covering all the bits of algebra that I've forgotten since school.
While some reviewers of this book say that the first 200 pages can be removed, you'd be missing out a clear introduction to the language and the concepts behind Processing, interspersed with some code. If you are already well versed with Processing or Java, this might be quite dull for you, but I personally found the writing fresh and interesting.
To sum up, this is almost certianly a great introduction to Processing and programming for artistic purposes, but if you are looking for a blow-by-blow reference to Processing in a paper form, you would be better off with the Reas & Fry book. This 800-odd page text is perhaps best for those without a firm background in programming, but are willing to learn. If that's the case for you, then this book is certainly one to consider.