To me it is a little bit strange to defer to the knowledge of an American, or any foreigner, about the Matter of Britain. But that is by the by, and Loomis evidently knows his stuff. This is a survey of Arthurian literature up to and including Malory; if you are interested in learning more about how the cycle assumed the form is has now, and finding out which of the earlier books is worth the trouble without having to read all of them for yourself, then it is for you.
Loomis' taste is good, as far as my knowledge of the subject goes, and his prose very readable. But this is a field where speculation has long run riot, and Loomis is no exception; he is sometimes close to coming unstuck in trying to identify the yet older traditions behind the books as we have them. Perhaps the Grail story is made up of strands of old Celtic myth, but the ones he identifies are relatively incidental things - like the upbringing of Percival. He can find no convincing antecedent for the Grail itself, or the quest, and it seems odd to overlook the obvious explanation that these are simply what they seem to be: a sort of popular Christian myth, parallel to the official teachings of the church and adapted to the culture of chivalry. Where did they come from? Well, do they have to have come from anywhere - anywhere, that is, but where we find them, in medieval romance?
And similarly in discussing Gawain and the Green Knight, though he rightly rates its literary qualities highly, Loomis seems unable to see the Christian wood for the pagan mythological trees. Again - and as Tolkien said - it is surely just what it appears to be, a story exemplifying Christian virtues, not heroic ones despite the general chivalric context.
I believe that the 'something' the Arthurian stories have, which causes them to haunt the popular imagination so much more persistently than any other other similar cycle in Western culture, is not to be be found behind the extant books - in history or mythology - but in those books themselves.