Before the museum, there was the Wunderkammer. This collection of oddities, exotics and curiosities, resembled the museum, in that it brought together artefacts from all over the world in one physical location. However, it lacked the thematic, edifying and classificatory intent of the museum.
Dunning's collection of essays seems to me to be a Cabinet of Curiosities, a Wunderkammer. Therefore, if you are hoping for an overarching theme or unifying rationale for this book, you are likely to be disappointed.
Even the title is wrong. 'Extremes: Reflections on Human Behaviour' doesn't seem to fit the collection (which covers topics such as the history of anatomical research, castrato singers, cannibalism, anorexia and religious fasting) and Dunning doesn't do a great deal of 'reflecting'. Dunning was a cardiologist, and this is reflected in his choice of topics, so if you're interested in the history of medicine, then you will probably enjoy it.
I originally read it because of my interest in Gilles de Rais. However, I found plenty of other things to interest me too. For example, I discovered that Edgar Allan Poe wrote a story featuring a character named Richard Parker, a cabin boy who is cannibalised on a life boat (the name Yann Martel consequently chose for the tiger in his novel, Life of Pi).
It's that sort of book - you're like to learn something that you didn't know before, but you may wonder if you really needed to know it.