This week's book giveaway is in the OO, Patterns, UML and Refactoring forum. We're giving away four copies of Refactoring for Software Design Smells: Managing Technical Debt and have Girish Suryanarayana, Ganesh Samarthyam & Tushar Sharma on-line! See this thread for details.
I would add that all interfaces in Java are implicitly abstract, anyway. So don't worry about looking for "abstract interfaces" in particular - just concentrate on understanding interfaces, period. At least as far as Java is concerned.