Sufism in Islam by Hamza Yusuf

The Scholars on Sufism

Sufism, Sufis, and Sufi Orders: Sufism's Many Paths by Professor Alan Godlas

The Interior Life by Dr. Seyyed Hossein Nasr

List of Sufi-related sources on the internet

The Sufi-Rumi Connection: Kabir Helminski, a sheikh of the Mevlevi order, talks about Rumi, Sufism, and their roots in Islam.

Tasawwuf 

www.tasawwuf.org

Fundamentals of Tasawwuf