I have read and enjoyed this book several times but I have reservations although the book trumpets itself as an account of 'mad' rulers:
'From Caligula to Stalin...offer(ing) a unique and pioneering look at the recurring phenomenon of the 'mad king' from the early centuries of the Christian era to modern times...the author contend(s) that mental health has played a determinant part in the making of history, where private traumas provoke the public policies of deranged statesmen. This controversial study makes for fascinating reading; it also offers a challenging new perspective on our understanding both of history and contemporary politics.'
Let's be clear the author tells some fun stories but really hasn't developed any coherent narrative or systematic theory. That would be impossible because the 'sources' for many of the stories he tells, like Seutonius, are themselves problematic. Just trotting out the old stories about Caligula making a horse consul just won't do. To many of the later examples like Henry VI in England or Carlos II in Spain have more to do with genetic implosion than 'madness'. Poor Juana Queen of Castile was more likely driven 'mad' by being treated as mad. I doubt if Gian Gaston, the last Medici grand duke of Florence was mad, he was a drunken voluptuary, but he made many sensible decisions. He just realised that, as the last male of his line, his wishes were irrelevant and everyone was just waiting for him to die so they could take over lands.
That doesn't mean that the stories told are not interesting. It was here I learnt about poor Christian VII of Denmark, the subject of the wonderful novel' The Visit of the Royal Physician' by Per Olov Enquist, which I later read. I probably understood more about Christian VII and his times from Enquist's novel, it helped that he could read sources in the right language. Clearly Dr. Green does not read Latin, Spanish, French, Swedish, Russian, Italian, Danish, etc. never mind do archival research in all those countries so a great deal of his 'research' is from, often dubious and very old, secondary sources.
I was most appaled by his attempts to coral into his 'theories' modern politicians or dictators like Hitler and Stalin. Such reductive simplicities, particularly where Hitler and Stalin are concerned only distort rather than explain their actions.
This book is very readable, full of interesting stories, but can only be treated as a jumping off point. To rely on it to 'understand' the biographies of any of these rulers, their actions or times would be a great mistake.