Tom Green's excellent study follows a growing scholarly trend to treat the hypothesis of an historical Arthur seriously, even if it means ultimately demolishing the case for a genuine hero of the same name. Nick Higham, for example, showed how the 9th-century 'Historia Brittonum' (attributed to Nennius) was put together with a particular political agenda in mind and so must not be relied on to accurately reconstruct post-Roman British history.
Unlike Higham (who accepted that there might possibly have been some Arthur-type warlord at the core of the Nennian construct) Green argues, I think persuasively, that there never was such a prototype historical figure but that the earliest sources (some contemporary with and others predating Nennius) make it clear, first, that Arthur was a mythological figure, defender of Britain from giants, monsters, witches and the like; and, secondly, that it is Nennius who first historicizes Arthur by pitting him against human adversaries (namely, the Saxons) and attributing to him a selection of mythological and genuinely historical battles. Those who instinctively felt that Arthur was more an archetypal hero than a flesh-and-blood warrior may now feel more vindicated.
If I have a criticism it's this: that Green's dense discussion frequently repeats itself, perhaps reflecting the fact that much of his material appeared as scholarly papers online. This is a shame as his message and arguments, while needing to be academically rigorous, also deserve to be more generally accessible. If potential readers can stick with it, 'Concepts of Arthur' is an inspiring read which does not disappoint those who want a satisfying contextualising of disparate evidence.
By the way, if you've come expecting discussions about Lancelot and Guinevere, Merlin, Camelot, the Sword in the Stone and the Lady of the Lake, forget it: most of these motifs (in the form that most of us are familiar with) belong to later medieval accretions and the fantasies of modern popular culture.