A very comprehensive, well-researched book about the origins of the Arthurian legend, its historical context, and evolution. Some of the chapters are specifically about certain characters or elements and how they evolved and became part of the Arthurian literature. For example, there is a chapter about Tristan and Isolde and how it was interpreted and adapted by various authors.
It also adds references to English translations for the myths and stories that are mentioned, which I found very helpful, and a brief who's who at the end that can come in handy in the absence of Norris's Arthurian Encyclopedia.