I am, perhaps, somewhat biased about this book. I've read it, probably, at least a dozen times: let's put it this way, my hardcover copy is falling apart. Clearly I'm a fan, not just of Arthurian fiction but of Marion Zimmer Bradley as well.
A life-long enjoyment of Arthuriana will teach you one very clear thing: there is no definitive story or Arthur, and therefore all interpretations are as valid as the next. The existence of Arthur can barely be proven, and his identity is certainly up for debate. The stories of Camelot, the Round Table, and certainly of Lancelot and Guinevere are all Norman-French additions to a tale set hundreds of years prior in the Dark Ages. Arthur is an enduring legend but, as we know him, mostly a legend nonetheless.
Bradley's story, then, of the tale of Arthur through the women who knew him, is no less valid an interpretation than any of the rest, and certainly a unique one. Rich with its own legends and myths, "The Mists of Avalon" begins with Igraine, and goes forward through the eyes of Viviane, Morgaine, Morgause, and Gwenhwyfar, each with their own perspective on what truly were momentous times in the history of Britain. As Arthurian myth, it stands on its own two feet as well as any other, with its tales of war, love, religion, loyalty and betrayal spread across and repeated through several generations, closing with the end of an era and the beginning of Saxon rule over the island.
It is also, however, a tale of one human woman, Morgaine, and her life: her beginnings, her path, her faith, her love, her choices, her mistakes, and ultimately, her will to survive. The lesson of "The Mists of Avalon" that I take with me each time I read it is this: we are born to strive towards something, and in the struggle, we sometimes misstep along the way.