How do I approach professors for reading courses?
I think the best approach would be to find out when the professor will be in his office and approach him and ask (or wait outside his class until class ends). I think he would appreciate having someone be interested in his work and maybe agree - after all, a university is meant for knowledge sharing right? 

You're right, if he is on sabbatical then there's nothing you can do. You will probably have to show that you are really interested and aren't there to waste his time.