"Refactoring" is next on my list, but I started with "UML Distilled", and am currently working on "Design Patterns", partly in preparation for "Refactoring" (and "UML Distilled" was partly in preparation for "Design Patterns").
Read Refactoring first than read Design Patterns(GoF).
http://www.amazon.com/Refactoring-Patterns-Joshua-Kerievsky/...