In addition, a really good book is the Design Patterns book by the gang of four. In this great book, they discuss how to do many tasks in programming while avoiding inheritance and/or modification of existing code while extending the functionality.

Anybody who is interested in object oriented programming should take a look at this, whether his language of choice has single or multiple inheritance (being able to read C++ and Smalltalk helps though).

The book is not for people who want to learn how to do object-oriented analysis and how to arrive at a good object model - the book only discusses how you can solve certain small tasks by applying a pattern/an idiom. Whether an object-oriented approach is fitting at all is a second question, of course.