Mystical Dimensions of Islam by Annemarie Schimmel was the first book I read about Sufism and I still have a great fondness for it. Annemarie Schimmel was a great scholar and also a practising Sufi. It is a beautiful and very thorough survey of Sufism, its history, its practices, and with special emphasis on Sufi poetry. This is an excerpt from the product description,
Mystical Dimensions of Islam presents, for the first time, a balanced historical treatment of the transnational phenomenon of Sufism—Islamic mysticism—from its beginnings through the nineteenth century. Through her sensitivity and deep understanding of the subject, Annemarie Schimmel, an eminent scholar of Eastern religions, draws the reader into the mood, the vision, the way of the Sufi in a manner that adds an essential ingredient to her analysis of the history of Sufism.
It is well worth having this book and reading it often.