The 2 reviews that precede mine are pretty accurate, so I'll make this short; in fact, I'll oversimplify the book.
The gist of the book is that money should be merely a unit of account. It should not be a medium of exchange or a store of value. Ideally, each community should have its own money and there should be no exchange of it. Rather, all local money should exchange at fixed rates into a unit of non-tradeable, non-lendable, non-spendable "international money" which then gets exchanged back into the money of another jurisdiction when you want to buy something there. Further, capital, surplus and savings are suboptimal, economically. They should be taxed to force the holders to spend them on goods and services. Debt should not be re-financed and should not be assignable. Trading debt causes lenders to be less vigilant. Rather, the lender should hold 100% of every loan and the borrower should pay it back (the author calls this a fiduciary relationship which is not accurate in the Anglo-American legal tradition; I cannot say what it would be called in civil law).
Sovereign debt cannot practically be repaid and sovereigns have no intention of doing so. They will repay when convenient and default when they need to. Currency used to be redeemable in something the sovereign could not control, effectively a debt, but in the 70's Nixon got rid of that and now we have fiat currencies, which illustrates the point. The only interesting question is how far the sovereign extends the prerogative to default with impunity. Its debts, its currency, its central bank, others? But, if we eliminate capital, there won't be any loans to go unpaid. If we adopt these principles, we won't have any financial crises. What we will lose, goes unaddressed.
Of course, I oversimplify, but not inordinately. The book is a difficult read as one reviewer before me noted. There is a good history section that was enjoyable.
It was interesting to see two Italians write this book. It is in many ways an Italo-centric view of the world, a world lived inside small walled towns.