The Mystics of Islam by Reynold A. Nicholson. Academic introduction to the origins of Sufism. In The Mystics of Islam, Reynold A. Nicholson draws lines to old Christian, gnostic, neoplatonistic and even the Buddhist influences from where Sufism origins. The book is rich on examples, poems and references to Sufi masters and writers. From the introduction:

The earliest Sufis were, in fact, ascetics and quietists rather than mystics. An overwhelming consciousness of sin, combined with a dread – which it is hard for us to realize – of Judgment Day and the torments of Hell-fire, so vividly painted in the Koran, drove them to seek salvation in flight from the world. On the other hand, the Koran warned them that salvation depended entirely on the inscrutable will of Allah, who guides aright the good and leads astray the wicked. Their fate was inscribed on the eternal tables of His providence, nothing could alter it. Only this was sure, that if they were destined to be saved by fasting and praying and pious works–then they would be saved. Such a belief ends naturally in quietism, complete and unquestioning submission to the divine
will, an attitude characteristic of Sufism in its oldest form. The mainspring of Moslem religious life during the eighth century was fear–fear of God, fear of Hell, fear of death, fear of sin–but the opposite motive had already begun to make its influence felt, and produced in the saintly woman Rabi‘a at least one conspicuous example of truly mystical
self-abandonment.

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