Early on in this book (page 21) the 'turn' gem is recommended. Later on in chapter 7 the book talks about ZenTest and autotest. Turn and autotest do not play well together. If you have the "require 'turn'" statement in test_helper.rb autotest will always report all tests passing - at least that is what was happening for me.
If this book ever gets revised it would be nice to warn people about this.
Hope this saves someone else some time, took me a while to sort this out.
Here is the tracker page for this issue: