Refactoring is the process of improving the existing system design without changing its behavior.
Besides Martin Fowler's seminal book "Refactoring - Improving the design of existing code" and Joshua Kerievsky's book "Refactoring to Patterns", are there any good resources on refactoring?
Working Effectively with Legacy Code focuses on dealing with existing code-bases that need to evolve to be testable. Many techniques are used in the book to accomplish this, and is an excellent resource for refactoring.
If you're looking for more than just code refactoring, you might find Scott Ambler's book quite useful: http://www.ambysoft.com/books/refactoringDatabases.html
http://www.refactoring.com/ might help you. They have a long list of methods here:
Joel's article Rub a dub dub shows you why you should refactor and not rewrite (but I guess you already knew that rewriting is a thing you should never do..)
Here are some Wiki pages about refactoring that explore various principles and guidelines.
Refactoring HTML is new and relatively good, you can guess what it covers :)
Other than that the two books you mention are the two I've used most, but Agile Principles is also very good.
There is a 'cheat sheet' for code smells here: http://industriallogic.com/papers/
I would recommend reading Working Effectively with Legacy Code, then Refactoring - Improving the design of existing code. Martin Fowler's book is more like a receipt book for me, it explains how. Working effectively with legacy code, explains the why in my opinion.
below is some other books relating to refactoring:
antipatterns refactoring software architectures and projects in crisis
refactoring in large software projects performing complex restructurings