Nowadays most programmers know about code refactorings.
What about refactorings of data structures, are there any good readings about it?
One paradigm that I can think of is the normalization process of a relational database.
Are there any other good examples?
xUnit Test Patterns: Refactoring Test Code is a good reference for refactoring your code to work well with unit tests. It's not exactly what your asking for but its a good reference to keeep on hand.
Much like code, data can be refactored by using an abstraction layer in front of it. Instead of using raw SQL to access your data, put a layer in front of it to abstract the queries. Not only will it let you refactor the data structure but it'll also let you completely change your data repository.
Refactoring Databases : Evolutionary Database Design seems to be a worthwile read on the subject, judging from the fact that it won the 2007 Software Development Jolt Productivity Award in the Technical Books category. I haven't yet read it though, so I can't comment on it personally.