Encyclopedia Britannica includes a very detailed article by Annemarie Schimmel on thistory of Sufism, its doctrine and the formation of Sufi Orders.
Encyclopaedia of Islam (2nd edition) includes a comprehensive article on Tasawwuf by Leonard Lewisohn, William Chittick, J.O. Hunwick, Jong, F. de. , and Radtke, B.
Oxford Islamic Studies Online encompasses over 5,000 A–Z reference entries, chapters from scholarly and introductory works, Qur'anic materials, primary sources, images, maps, and timelines, providing a first stop for anyone needing information and context on Islam.
Available at McGill:
The Directory of Free Arab Journals (DFAJ) is a directory of Open Access journals published in the Arab World. The website is maintained by a group of OA activists in the region, and includes 319 journals from 150 publishers in 18 different countries.
Allama Iqbal Poetry کلام علامہ محمد اقبال gives access to the largest Poems & Ghazals Collection of Kalam-e-Iqbal in Urdu, Roman Urdu & English Translations.
Dar al-Masnavi is the activity of the American Institute of Masnavi Studies (AIMS) on the Internet. It is run by Dr. Ibrahim Gamard. It is a comprehensive site on Rumi's Mathnawi.
The Muhyiddin Ibn 'Arabi Society publishes on its website news and announcements of events, and provides information about publications on Ibn 'Arabi. It has over 150 translations and articles republished from the Society's Journal and other sources.
Sufism -- Sufis -- Sufi Orders: Sufism's Many Paths is curated and maintained by Dr. Alan Godlas, University of Georgia.
Abbas, Shemeem Burney. The Female Voice in Sufi Ritual: Devotional Practices of Pakistan and India. Austin, TX: University of Texas Press, 2002. Electronic resource.
Ahmad Sirhindi, and Arthur F Buehler. Revealed Grace: The Juristic Sufism of Ahmad Sirhindi (1564-1624). Louisville, KY: Fons Vitae, 2011. Print.
Bang, Anne K. Sufis and Scholars of the Sea: Family Networks in East Africa, 1860-1925. London; New York: Routledge, 2003. Online.
Buehler, Arthur F. Sufi Heirs of the Prophet: The Indian Naqshbandiyya and the Rise of the Mediating Sufi Shaykh. Columbia: University of South Carolina Press, 1998. Print.
Chittick, William C. Ibn ‘Arabi: Heir to the Prophets. Oxford: Oneworld, 2005. Electronic resource.
Chittick, William C. Imaginal worlds: Ibn Al-‘Arabi and the Problem of Religious Diversity. Albany: State University of New York Press, 1994. print.
Corbin, Henry. The Man of Light in Iranian Sufism. Boulder: Shambhala ; distributed by Random House, 1978. print.
Curry, John J. The Transformation of Muslim Mystical Thought in the Ottoman Empire: The Rise of the Halveti Order, 1350-1650. Edinburgh: Edinburgh University Press, 2010. Electronic resource.
De Bruijn, J. T. P. Persian Sufi Poetry: An Introduction to the Mystical Use of Classical Persian Poems. Richmond, Surrey: Curzon, 1997. Print.
Ernst, Carl W. Ruzbihan Baqli: Mysticism and the Rhetoric of Sainthood in Persian Sufism. Richmond, Surrey: Curzon Press, 1996. Print.
Ewing, Katherine Pratt. Arguing Sainthood: Modernity, Psychoanalysis, and Islam. Durham, NC: Duke University Press, 1997. Print.
Le Gall, Dina. A Culture of Sufism: Naqshbandis in the Ottoman World, 1450-1700. Albany: State University of New York Press, 2005. Print.
Geaves, Ron. The Sufis of Britain: An Exploration of Muslim Identity. Cardiff: Cardiff Academic Press, 2000. Print.
Green, Nile. Making Space: Sufis and Settlers in Early Modern India. New Delhi: Oxford University Press, 2012. Electronic resource.
Harder, Hans. Sufism and Saint Veneration in Contemporary Bangladesh: The Maijbhandaris of Chittagong. Abingdon, Oxon; New York, NY: Routledge, 2011. Print.
Hoffman, Valerie J. Sufism, Mystics, and Saints in Modern Egypt. Columbia, S.C.: University of South Carolina Press, 1995. Print.
Jironet, Karin. Sufi Mysticism Into the West: Life and Leadership of Hazrat Inayat Khan’s Brothers 1927-1967. Leuven, Belgium: Peeters, 2009. print.
Karamustafa, Ahmet T. God’s Unruly Friends: Dervish Groups in the Islamic Later Middle Period, 1200-1550. Salt Lake City: University of Utah Press, 1994. Electronic resource.
Katz, Jonathan Glustrom. Dreams, Sufism, and Sainthood: The Visionary Career of Muhammad Al-Zawâwî. Leiden; New York: E.J. Brill, 1996. Print.
Knysh, Alexander D. Ibn ‘Arabi in the Later Islamic Tradition: The Making of a Polemical Image in Medieval Islam. Albany: State University of New York Press, 1999. Print.
Kugle, Scott Alan. Sufis and Saints’ Bodies: Mysticism, Corporeality, and Sacred Power in Islam. Chapel Hill: University of North Carolina Press, 2007. Electronic resource.
Laffan, Michael Francis. The Makings of Indonesian Islam: Orientalism and the Narration of a Sufi Past. Princeton: Princeton University Press, 2011. Electronic resource.
Lewisohn, Leonard. Beyond Faith and Infidelity: The Sufi Poetry and Teachings of Mahmud Shabistari. Richmond, Surrey: Curzon Press, 1995. Print.
Lewisohn, Leonard. The Heritage of Sufism. Oxford; Boston: Oneworld, 1999. Print.
Mason, Herbert. Al-Hallaj. Richmond, Surrey: Curzon Press, 1995. Print.
Millie, Julian. Splashed By the Saint: Ritual Reading and Islamic Sanctity in West Java. Leiden: KITLV Press, 2009. Electronic resource.
Muedini, Fait. Sponsoring Sufism: How Governments Promote "Mystical Islam" in Their Domestic and Foreign Policies. New York: Palgrave, 2015. Print.
Nasr, Seyyed Hossein. Three Muslim Sages: Avicenna, Suhrawardi, Ibn ‘Arabi. Delmar, N.Y.: Caravan Books, 1976. Print.
Netton, Ian Richard. Sufi Ritual: The Parallel Universe. Richmond, England: Curzon Press, 2000. Print.
Pemberton, Kelly. Women Mystics and Sufi Shrines in India. Columbia: University of South Carolina Press, 2010. Electronic resource.
Qureshi, Regula. Sufi Music of India and Pakistan: Sound, Context, and Meaning in Qawwali. Cambridge; New York: Cambridge University Press, 1986. Print.
Ramey, Steven Wesley. Hindu, Sufi, Or Sikh: Contested Practices and Identifications of Sindhi Hindus in India and Beyond. New York, NY: Palgrave Macmillan, 2008. Print.
Ridgeon, Lloyd V. J. ‘Aziz Nasafi. Richmond: Curzon, 1998. Print.
Seesemann, Rüdiger. The Divine Flood: Ibrahim Niasse and the Roots of a Twentieth-Century Sufi Revival. Oxford: Oxford University Press, 2011. Electronic resource.
Sells, Michael Anthony. Mystical Languages of Unsaying. Chicago: University of Chicago Press, 1994. In Print.
Sirriyeh, Elizabeth. Sufis and Anti-Sufis: The Defense, Rethinking and Rejection of Sufism in the Modern World. Richmond, Surrey: Curzon, 1999. Print.
Weismann, Itzchak. Taste of Modernity: Sufism, Salafiyya, and Arabism in Late Ottoman Damascus. Leiden; Boston: Brill, 2001. Electronic resource.
Winstedt, Richard. The Malay Magician: Being Shaman, Saiva, and Sufi. London: Routledge and K. Paul, 1951. Print.
Zelkina, Anna. In Quest for God and Freedom: The Sufi Response to the Russian Advance in the North Caucasus. Washington Square, N.Y.: New York University Press, 2000. Print.