They really are two different but complementary books. he BB is really about what to do. Going over concepts like what a 30-60-90 day mailer and how his data shows the benefits over a 45 day mailer in terms of sheer orders, but how a 45 day mailer activates more lazy customers. My data showed slightly different returns, and even that varied between 5 shops, so KK really isn’t the gospel, but it is well worth the money I spent to buy it. It also goes over his letter in depth. That failed miserably for me in 3 separate tests, but then again a lot of operators have had great success with it.
Gun to the Head is more about what and how to write good pizza copy. If you have ever taken any copywriting classes, you would see a lot of the same advice. Basically it is a manual on how to get the maximum impact from your written marketing materials.
Both are kind of gimmicky. Look at the sales letter/web site he has for the BB. Does that make you want to buy it? In general it does, and he teaches you to do the same thing to your pizza customers.
All in all, they are well worth the money I spent on them, but if you haven’t read a business book in a decade, then there are a lot of books out there that are going to get you a much better return.