It's a nice article about Encapsulation by Martin Fowler. Fowler is one of leading authors in the design area. His (with Kent Beck and other coauthors) book Refactoring: Improving the Design of Existing Code is a classic. It's rated number 3 in the JavaLobby poll of most useful Java books. It also has 4.5 stars after an amazing 113 Amazon reviews.
It has about 70 named "refactorings" you can do on your code, each in its own section. Unlike some books (Design Patterns by Gamma et al), this is quite readable. Altho I always learn something when I open it, that doesn't happen very often - perhaps it belongs near the porcelain throne to be read in the small sections that it's built from.