This book was fascinating because it gives a clear timeline of the history of the ancient Britons of Ynys Witrin based on other historical accounts (and helps to interpret the truths of these sources as well). It is tremendously hard to find this information elsewhere in relation to the Tor and ancient Welsh mythology.
Most accounts found online are jumbled and written based on the perspectives of one people or another and tales handed down, but don't look further back in time. This book does.