First, thanks for the compliment on the book. I'm glad you are liking it. I hope it helps.
Second, sorry for taking so long to respond.
Third, you are exactly right. It should be Buttons.Back. I'll update the errata to have the same text I have for Chapter 9 put into Chapter 7:
With the change from XNA Game Studio 1.0 Refresh to XNA Game Studio 3.0, Microsoft included a enumeration type for buttons. In the original book we created our own type InputHandler.ButtonType. The built in enum type is simply Buttons. The code should reference Buttons instead of InputHandler.ButtonType.
Thanks for bringing this to my attention!