That would seem to be the ultimate acid test. Unless you are reviewing it and saying, "This is a fun read, and worth buying as any fiction book would be for the sheer fun of reading it." In which case, that is ok.
But my guess is that most millionaires are busy making money, instead of writing books. UNLESS they are making their money from writing books. In which case, wouldn't the book simply say, "Become a millionaire by writing books that purport to tell others how to become millionaires."
I'm gonna pass. Too many of these books seem to be written by people who make their money selling books on how to make money.