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.
Fundamentals of Tasawwuf