[mschwar99]

I know you asked for a book, but as a fellow self taught person I remember this video being really valuable to me when trying to get my head around OOP.

http://video.google.com/videoplay?docid=-2058469682761344178...

It is an older video of Dan Ingalls of Smalltalk fame explaining objects. I clearly remember a large light bulb going off in my head as he walked through his analogy of a hospital described as an object.

This certainly isn't comprehensive, but if you are stuck on the concept its helpful to hear someone talk it out.