According to the Wikipedia article about him, he also created the idea of Design by Contract , involving preconditions, postconditions and invariants, and DbC is built-in to Eiffel. DbC is now supported in many languages or in their libraries.
Scott Ambler's The Object Primer was also good, also read some years ago.
IBM developerWorks article by him about the topic:
Just checked, it is still available, though the page at the link below says there is a newer version (linked on same page):
Update: Just remembered, when I was checking out Eiffel some years ago, I had read up on success stories about it. One very interesting one was about how HP used it to create a printer driver for a printer, after attempts using another more mainstream language had worked, but the driver had a lot of bugs. IIRC, the article  said that with Eiffel, the new version of the driver was created a lot faster, and also had a lot less bugs.
 Found the article from a Google search: