This book was not written by the original author; rather, it is an independent publication that provides a detailed summary of the original work.
The book delves into two historical "ends." The Bronze Age Collapse occurred around 1200BC when civilization was knocked back on its heels over about 50 years, and it took around 400 years to recover to where humanity had fallen from. When Rome fell around 400AD, it ushered in another period of 500 years or more of a radically different world than had preceded it. Dan examines the various causes of both collapses, as well as how difficult it is to pinpoint just one factor that caused such a profound shift. Sometimes it's the result of some things coming together.
He then devotes a few chapters to examining potential reasons for a future collapse that might affect present civilization, leading to another. The biggest competitors are plague and war. Then he concludes with reflections on whether or if, centuries from now, someone will be looking back on our period in the same way that we are presently looking back on pre-fall civilizations from prior eras.
It's hardly the most optimistic of books. Because mankind tends to repeat past cycles rather than learn from them. And just because we haven't had a significant collapse in the previous thousand years doesn't guarantee we won't in the future. And given the rate at which some of them occur, it's likely that some people may witness both the before and after in their lifetime.