Well this isn't shelving, but my OH made a simple cat tree that they *LOVE*, really simply with just bits of old shelf and 2x4, but it could just as easily be shelves built out of the wall:
They now climb most of the way up the tall straight bit of wood at the front, hanging on for dear life with their claws. Really good for scratching, climbing and sleeping!
I'm sure I've seen a website linked on this forum of houses basically designed around cat shelves.
Have you tried just googling cat shelves? That'll give you a good idea to start with!