This is a very interesting mix of an academic paper and a history book about not exactly forgotten but rather ignored genderqueer identities through different times and cultures - and everyone can learn something new. 
It gives a very broad insight into the topic and tries to balance the focus between historical accuracy and bringing forth proof while shifting the narrative from our modern standpoint towards something more suitable to the time period. The premise is something of a "we can't say that they were trans if they did not have the resources and vocabulary available that we now possess but neither can we say that they weren't and therefore we both need to broaden up the term trans* and need to see them in their historical contex to realize that they really were genderqueer".
Apart from specific people Kit Heyam also writes about the construction of gender and its stereotypes in different cultures.

While the explanations tend to get a bit repetitive, the tone is very inclusive and the author definitely wrote this for both academic and casual reades. Overall a really good book to broaden ones horizon and to learn about a few of the many(!) genderqueer/trans* people which have existed in history befoer us. I'd say it's more of an introduction to the discussion and there are definitely books which talk about specific parts more in depth - many of them are cited and recommended by the author in this book, too.