Why an Age of Tyrants? Well so said Gildas (the title quote above) after Jerome's cataloguing of Britannia as "a province fertile in tyrants". Unlike many books about 'Dark Age Britain' this is not a book in which the historian is pushing a particular thesis, and it is all the better for that. Too much of the historigraphy of this fuzzy period in British/English history has been characterized by academics (never mind all the others in search of you-know-who and his band from Camelot) shouting at each other. Snyder's book is packed with data and information from sites across the country, which is very useful if you are a layman like me who doesn't get to hang out with archaeologists when they write up their reports). I particularly enjoy the gazetteer section on sites of sub-Roman Britain. And yes, he does discuss Arthur (and Merlin) eventually in an appendix!
Snyder opens with a short section describing the known history of Britain in the 4th century up to about 410. The second section begins with a discussion of the literary evidence available for 400-600, leading into a series of chapters discussing the meaning of terms used in the literature: 'Britannia', 'patria', 'Britanni', 'cives', 'reges', 'tyranni' and others.
Section three considers the archaeology, before section four supposedly synthesises the previous two sections. Personally I totally failed to see the argument presented, if indeed there was one. Snyder declares himself in sympathy with the "positivists" trying to reconstruct some sort of detail as opposed to the "reductionists" who say that nothing can be said for certain, but I must have missed his conclusions altogether.