From “The Sanusi Letters: A checklist”, by Knut S. Vikør. Available freely online.
“The obvious importance of the Sanūsī order has made it one of the better studied movements of nineteenth-century Islam. Traditionally, however, the movement has been known mostly from external sources, in particular French colonial observers, and from overviews such as those of E.E. Evans-Pritchard and Nicola Ziadeh. There exists, however, a rich body of material from inside the order. They include the scholarly works of its masters, but also non-literary material such as letters of various types. In recent years, more and more of these have been published in various works on the Sanusiyya. They are, however, spread throughout many publications and it may be hard to obtain an overview of them. The present list is a survey of known letters to and from the Sanūsī order.”