Great question, I'm also interested in the authors' answer.
From my experience, decoupling the layers in an application is the key to making your code testable. For instance, in an MVC style web application, don't pass the HTTPServletRequest outside of your Controller (e.g. action class in
struts) or you will make it more difficult to unit test your Model etc.
Really interested to hear if you book addresses such issues.
Regards
Paul