What’s confusing to me is that (1) I see ads for stuff like this all the time, and (2) there actually were people who made a lot of profit in the global financial troubles in 2008 (as written up by Michael Lewis in The Big Short). So I don’t see a joke here. Is there one?
Comics I Don't Understand 
A true financial collapse destroys the money system. So there is no money to profit with.
I interpreted it to mean that you can profit by selling a book pretending to have a secret but the real way to get rich is selling a book to the gullible.
I recall when there were constant ads for Free Seminars on how to make big money on foreclosures. The ads always showed pristine upper-class homes, and the promise seemed to be you could buy them for pennies on the dollar by showing up at an auction. I’d picture Snidely Whiplash before a roomful of eager pupils dressed like him, learning to properly laugh and twirl the mustache while evicting old ladies.
The pitch is still being made today, usually with the term house flipping, although now the odds are even longer with big players bidding up prices.
I suspect that the joke here is that this book is a big seller because people are expecting a major financial collapse.
Back in the days of classified ads, a ploy was to advertise a guide to making money stuffing envelopes. The guide would say to take out a classified ad, advertising a guide to making money stuffing envelopes.
Could the joke be the sheer size of the book?