The New Case for Gold by James Rickards, author of the New York Times bestselling author of The Death of Money. 

This book is a guide to how gold used to be the basis for all financial systems and was decoupled from fiat currency. It also explains gold historically, and it's use in investment. 

As David Stockman says- when the monetary system fails, there will be a flight to the only money that's left in the system....