I've recently started to read the best book on object oriented programming. There are other good books that deepen your understanding of OO like "Refactoring" and "P of EAA" from Fowler, "Design Patterns" from the GoF, the McConnell books, "The Pragmatic Programmer", the OO books from Robert C. Martin and some others. All recommended to read. But the best I've read on OO in the last 15 years is Domain Driven Design from Eric Evans. There are a lot of deep insights about how to build your domain model and structure your objects and classes in that book. Highly recommended.