Hard to say what you mean by 'properly'. Most photoshop users can use it competently, few are experts. I've heard it said that very few users even scratch the surface of what photoshop can actually do. Myself, I've just picked up lots of things from other people and reading.
I've got a huge book on photoshop, but even that is very much focused on doing very specific things (touching up photos.) All depends on what you want to do.