Ernst does explain some things quite well, though, such as the development of Sufi orders, the role of the shaik to the disciple, art, dance, and meditation practices. Particularly fascinating is the brief discussion on the relationship between Islamic fundamentalism and Sufism. Sufism is viewed by the fundamentalists as a type of heretical pseudo-Islam, and the Sufis have been persecuted and killed in many places by over-zealous fundamentalists. What Ernst's book shows is that many of these supposedly heretical practices are actually rather traditional expressions of Islamic mysticism and spirituality, with roots stretching back quite far.
Socio-historical perspectives aside, though, you won't learn much about the spirit of Sufism. For that, I would recommend Annemarie Schimmel's _Mystical Dimensions of Islam_ as an excellent place to start. Then, for a greater insight into socio-historical issues, you may continue with reading Ernst's book.