Nudibranch references?

My dad is into diving in a big way, and has started taking pictures and trying to ID everything he's photographed. (Sometimes this takes him months)

He's struggling to find good nudibranch references. He's found one book that's out of print and used copies go for $150. I was wondering if folks here had good website links or recommendations for less expensive books?

Also, if he were to join here, would people be able to ID wild nudi's from pictures? Or is knowledge limited to things normally found in marine aquariums?

Any help would be most appreciated! Since his dive buddies didn't know where to look, he was hoping my aquarium buddies might!

Thanks a lot folks! My dad will be thrilled. I will suggest he post things here. After hanging around my house all Christmas, I think he might be considering a marine tank, so bringing him here would be a natural step in that direction.