Baillie's premise is that every so often the planet passes thru clouds of debris, cometary and otherwise but mostly the incoming rocks if big enough are seen as "comets". For those of us who read history rather than have it taught to us there has long been a question as to how much of Europe and especially Britannia descended so far so fast after Rome pulled back.
Baillie's is not the first time I have seen the conclusion that SOMETHING in the early to mid sixth century (AD) knocked much of Europe off it's civilized pins. It is the best most carefully thought out one I have read though.
Baillie is/was primarily a Dendrochronologist or a tree-ring historian in a more hard science view than many historians. He first saw THERE that there were times of stunted growth hemisphere wide if not more and went looking for a reason. This book details that search.
the first half is a bit dry in that he is a hard science scientist describing his on rather technical field. The latter 2/3's to half is much more open talking about how he was approached independently by other people outside his field who had come to the same conclusions, and talking about things like 'killing fogs' and 'The Concept of the Wasteland' brought on by major die-offs of but human and animals in areas where tectonic out-gassing had swept ashore poisoning everything in its path with a set of lungs.
His detail showing many of the Biblical Plagues to be explainable is amazing!
I found it worth my time, your experience may be different.