The Pragmatic Programmer
Hunt, Andrew and Thomas, David. The Pragmatic Programmer: From Journeyman to Master, Addison Wesley Longman, Inc., 2000.
Eiffel Webpage
Building bug-free O-O software: An introduction to Design By ContractTM, Eiffel Software Inc., 2002. Accessed 24 September 2002. <http://archive.eiffel.com/doc/manuals/technology/contract/page.html>.