This book is a fascinating look at a period of history I knew very little about - and then goes even deeper.
Many people know the basics of the story of Genghis Khan, but what I hadn't realised is what happened after he died. The short answer is: his daughters were a lot more competent than his sons, and more than that, it was the female descendents of his line who kept coming back to try and restore stability to the empire.
However, the name of the book comes from the fact that while most Mongol history was passed by oral tradition, there were written documents, including a work titled "The Secret History". Yet, while examining that for his previous book, Weatherford discovered that a number of references to the queens of the Mongol empire were cut out or otherwise removed. This book is the result of his research into what we know about them - fortunately, the surrrounding countries often had relevant records.
Weatherford works more or less chronologically, focusing on major figures, and explaining how they relate to each other (a very complicated topic.) I deeply appreciated the genealogical charts and maps, though I wish there'd been an index of where they were in the front of the book.
One other thing I really liked about this is that Weatherford is an anthropologist by discipline rather than a historian. That gave many aspects of the book an interesting perspective: he's detailed, thoughtful, and respectful of Mongol cultural and religious traditions, taking time to explain how they fit into the larger beliefs and practices of the community. (Especially around things like how family ties and interactions: for example, younger sons could speak to more senior wives, but not to the wives of the sons/nephews/etc. younger than them.)