The Journal of Object Technologies (JOT) has been publishing a wonderful series of articles by Anthony J.H. Simons on "The Theory of Classification" (a sort of primer on object-oriented type theory). Since the site of the journal does not have a page with the complete list of the links to the articles, I will provide one below:

• Perspectives on Type Compatibility
• The Scratch-Built Typechecker
• Object Encodings and Recursion
• Object Types and Subtyping
• Axioms, Assertions and Subtyping
• The Subtyping Inquisition
• A Class is a Type Family
• Classification and Inheritance
• Inheritance and Self-Reference
• Method Combination and Super-Reference
• Adding Class Types to Object Implementations
• Building the Class Hierarchy
• Template Classes and Genericity
• Modification and Objects like Myself
• Mixins and the Superclass Interface
• Rules of Extension and the Typing of Inheritance