Best book about working with legacy codebases?

I often have to deal with old spaghetti code at my company, so I was looking into buying a book to help me with this. These two seem to be widely recommended:

• "Refactoring: Improving the Design of Existing Code" by Martin Fowler
• "Working Effectively with Legacy Code" by Michael Feathers

If you could buy only one of the two, which one would you get? Which one should I start with?

Or please do share if you have a better recommendation.