The author (d. 412AH) is one of the most prolific early authors in Tasawwuf, second only to al-Harith al-Muhasibi according to Ibn al-Jawzi. His works present a very penetrating insight into the developments in Tasawwuf by his time.
Besides one or two works in hadith, he mainly wrote on various aspects of Tasawwuf. His Arba’in on the issues related to Tasawwuf was published by the Da’irat al-Ma’aarif, Hyderabad, Deccan. Hafiz Sakhawi has authored a takhrij of this work that has made it all the more valuable.
He has also authored al-farq bayn ‘ilm ash-shari’a wa ‘ilm al-haqiqah, which proves that by his time, the idea of the difference between shari’a and haqiqah was already in currency. This book is still in manuscript form (in my limited knowledge) somewhere and I would love to get my hands on it as the title promises it will be a revealing read.