A solid, readable introduction to Sufism. His dismissal of the idea of Sufis as a proselytizing vanguard was, to me, unexpected. It seems like Nile Green in "Sufism: A Global History" was pretty keen on that theory.
My only criticism is the structure felt a little sloppy at times, like his train of thought was frequently off the tracks. As readers we've been conditioned to expect (with good reason!) that ideas within a paragraph, for example, will be closely related to each other, and as the author moves from sentence to sentence to develop these ideas, they will flow from one into the other in a logical manner. I frequently found myself re-reading sections of this book because I felt Ernst was just going from one idea to the next – within a paragraph – with very little, if anything, threading them together.
All in all a good book, though, and I'd recommend it to someone interested in a serious but not stilted introduction to Sufism.