I've had spotty luck with the Mammoth series before; some are quite decent, some are obvious hack jobs, and one or two are just loony (the Mammoth Book of Jack the Ripper pops to mind). This one is divided into three rough sections, of unequal size and value.
The first attempts to break down the "historical Arthur," who Ashley defines first as "the war-leader of the Britons at Badon Hill" and only second as "the guy who Geoffrey of Monmouth was talking about." This results in a pretty thorough chase through obscure Breton king-lists, Nennius, the Ten Battles (fifty pages on them alone), the Welsh Triads, and so forth until he gets to a list of twenty, count-em, suspects. These range from Lucius Artorius Castus (the only Roman commander named 'Artorius' known to have served in Britain) to Arthwys ap Meurig (the king, perhaps, of Gwent in the seventh century, unless he wasn't). Ashley quietly plumps for an Arthur based in Gwent or Powys, but argues that Geoffrey's "Arthur" is a composite of five or six British leaders with mythic elements from Alfred and Aethelstan, and constructs a perhaps over-delicate genealogical lattice-work with which to argue that the victory at Badon was a coalition victory under a king of Dyfed named Agricola or Aircol, with one Vortipor/Gwerthefyr as the primary commander and possible "dux bellorum." This is about as good as things get without getting into Deep History. If this section has a flaw, it's probably best highlighted by Ashley's nervous-making habit of citing Laurence Gardner's Bloodline of the Holy Grail without using the words "barking mad." I'm certainly not an expert in post-Roman British chronicles, so for all I know, Gardner's research into the political-military complexities of the Saxon frontier is actually a model of meticulous restraint -- but I doubt it. Ashley does do a good job of highlighting when some fruity speculation is Gardner's and Gardner's alone -- the first time it appears. By contrast, he is politely dismissive of Geoffrey Ashe's various enthusiasms, and wisely so.
That takes us to page 306, where we begin about 200 pages of primer on the Arthurian Cycle, beginning with some potted history of the twelfth century and then into the various versions of the Tristan, Lancelot, Perceval, Galahad, Merlin, and other sub-cycles through the 14th century. This is excellent stuff, well presented; the Grail section is remarkably free of utter crazitude, although again it's no substitute for a specialist work on the topic like Richard Barber's The Holy Grail (the best single book on the topic). Then a short chapter on Malory, and another brings us up to Tennyson and the Pre-Raphaelites. This is all clear and relatively straightforward; it's also stuff I probably could have assembled myself from my bookshelves -- but not in only $13.50 worth of my time. We close up with sixty pages of Arthurian novels, narrowly defined (no That Hideous Strength or Drawing of the Dark), and ten pages of Arthurian movies -- this is all pretty disposable stuff. Its lack of attention to poetry means only tangential discussion of Eliot and (what's worse) none at all of Charles Williams.
Finally, we have about ninety pages of decent Arthurian Cycle "Who's Who" and gazetteer, more historically minded than Phyllis Ann Karr's wonderful (but primarily literary) Arthurian Companion and therefore of some good utility despite its relative brevity. A solid index in microscopic type concludes our program.
Ashley has read, and widely, all across the topic; for example, he cites John Hughes' far from seminal work Arthurian Myths and Alchemy in passing while discussing Malory and the court of Edward IV. Ashley cites Littleton and Malcor fairly, although space (and his inherent charity, perhaps) prevents full attention to their "Sarmatian thesis." He cites both Keys and Baillie on the Catastrophe of 535, and links it (too sketchily) to the discussion of the Waste Land. He even notes the possible ties between Amlawdd Wledig (from "Culhwch and Olwen") and Hamlet, although he (probably rightly) dismisses them. He misses one or two Arthurs from the fringe of the fringe -- there's no discussion, for example, of W.A. Cummins' dotty theory that King Arthur was actually a Bronze Age Wessex Culture monarch who built Stonehenge. (I'm hesitant to consider that particular absence a flaw.) Out there on the edge he does misstep occasionally; his brief discussion of St. Brendan badly confuses Brendan with Bran, both of whose immrama are relevant to the Arthurian mythos.
But on the whole, minor notches in the Sword of Strange Straps aside, this is an excellent one-volume compendium of Res Arthuria. The movie list is by far the weakest section; the various side-by-side comparisons of the various Cycles is probably the strongest, with the Gildas-to-Geoffrey section on "historical Arthurs" a close second. If you're more interested in post-Malory Arthuriana, try Norris Lacy instead. But if you need one good book on King Arthur, with a strong concentration on the pre-Galfridian material and the relevant historical background, this is probably the one to get -- you certainly won't beat the price.