The Processing Tutorials Page?

The sections that I would be most interested in I have already read because they are pulled directly from the book I am reading. Some four or five of those tutorials are out of this book.

Yes, but there is an Objects tutorial which should be about the classes that you wanted to learn about.

If that's a Reference you are looking for, it is right on the Processing site...
If that's tutorials you look for, indeed calsign's link is the way to go. It isn't obvious to beginners, but classes are just a kind of blue-print to make objects, so the two are related... One frequently found name is also OOP, or object-oriented programming.
You can also search for tutorials for classes in Java, since they are the same thing.
Lastly, the  Wiki is always worth a visit, as a general source of wisdom on Processing.

I would also like to recommend the Processing Vimeo Tutorials by Jose Sanchez.

Among other things they explain classes quite clearly.