The original paper on hush is [HUSH].
A veritable catalogue of object-oriented
applications can be found in [Harmon93].
A classical paper on class design is [JF88].
For the Law of Demeter, consult [LH89].
The notion of abstract systems
was introduced in [Henderson93],
which also gives a good account of a
formal approach to object-oriented design.
For an introduction to formal methods and Z,
For object-oriented extensions of Z, see [Stepney].