I suppose mine was an older sibling's school book. I made the mistake of loaning it out. Though it is on Project Gutenberg.

I think the details between the fall of Antiquity and the Middle Ages transitioning to the Age of Enlightenment were different beasts.

Quite simply, The Ancient world did slide back into a political authoritarianism that was a collapse of the belief of moral progress. Which the Middle Ages were a solution, basically oligarchy stabilized as monarchy. Which proved to be foundational to the world that rose from it. We still tend to see ideals as absolutes. Consider the ideologies following monarchy. Even today, each pole sees itself as right and the other as wrong.

There might be a split between the culture of monotheism and the civics of democracy, but it's not like there has been a return to the pantheism/theistic multiculturalism that gave rise to democracy in the first place.

The French and the Americans might have rejected monarchy, but it was technology, specifically the technology of war, that finally buried it, in WW1.

The future is a continuation of the past, until it becomes a reaction to it.

Our own age has some enormous blind spots as well. Such as treating money as a commodity to mine from society, rather than the social contract enabling it.

It's politics all the way down, but it's economics all the way up.