Not much of the book is actually about Stonehenge directly, but he looks at myths and legends that have been preserved in the medieval stories such as the Mabinogion and Saints lives which Tolstoy believes relate to the Omphalos (centre or navel) of Britain which he argues is what Stonehenge was believed to be. Although most of the sources he looks at are Celtic (Welsh and Irish), Stonehenge was built in the Neolithic age long before Celtic culture arrived in Britain. He convincingly argues that it was quite likely that Celtic people would have taken on some of the beliefs of their predecessors and the plausibility of traditions and rituals surviving over long periods of time although I'm not sure he made a case for the specific myths he was examining being derived from pre Celtic times.