I ordered this book after picking up debates on it on various blogs talking about ashtanga yoga - although the author addresses modern yoga as practiced in the west in general, it is perhaps ashtanga whose myths are most challenged by the book.
I am an ashtanga student myself, practicing almost daily - and it's easy for me to see how the gymnastic origins of what we do, as postulated by Singleton, is completely credible. Having seen yoga practiced by saddhus in India (their practice is also covered in the book - then, as now, they were seen as outside of mainstream Hinduism and viewed with disdain or suspicion) it is clearly different (and, I would suggest, in some cases involves a whole other level of commitment). For some though, the idea that they are practicing something ancient, found in ancient texts and passed down from a guru in a cave in Tibet to Krishnamacharya, and then to Jois, is important and makes what they do more than just exercise. What actually makes it more than just exercise as usually understood these days is probably the breathing more than anything else - timing the movement with breathing. That too though, is, according to the book, borrowed from western exercise systems of the nineteenth and earlier twentieth centuries. I don't care much personally, as I find all the cod-sanskrit and po-faced spiritualism of some in the ashtanga scene more than a bit tedious (and, to be fair, from what I've seen of Jois himself, he never took it so seriously). A change in approach may be required though from those who currently insist, with a straight face, that bending over and touching the floor is veda-inspired and a step on the road to enlightenment.
Great book anyway - well-researched (including interviews with a number of people who were students of Krishnamacharya), balanced (although some will see an agenda, he doesn't make assertions he can't back up with empirical evidence, and is generous in giving some of the stories told about the origins of modern yoga, such as the 'yoga korunta' being eaten by ants, more of the benefit of the doubt than they probably deserve), and, for an academic text, easy to read. Lots of very interesting photos comparing yoga poses and the systems which the thesis says they are borrowed from.