From my experience (Which is small, as it's limited to 3 years with PHP), OOP is still very much linear. There's no way to escape that with traditional computing abilities.
As for a book recommendation - I don't really know many books that lay out OOP...
There are very few books on Ruby on shelves as of yet, so you'll be hard-pressed to find one with a Ruby focus.
http://en.wikipedia.org/wiki/Object-...ed_programming is a Good reference on OOP, and I'd suggest reading it before purchasing anything.