I think Ben Fry's Visualizing Data is the best book to learn Processing, bar none.  Otherwise, the online intro would be nice to go over while waiting for the book to come.  

There's probably no need to go through any rigorous design process at first, which is the goal of this language.  As such, the language itself is very forgiving, so you can learn a lot by tweaking the online examples.  See if you can find an example the closely resembles the part of project you like to build, then modify parts of it to understand how it works.  Then paste some parts from another example you tweaked on top of it, just like a collage.