An exellent translation of the letters of Sheikh ad-Darqawi by Titus Burckhard. Although this book is only a partial translation and there does exist another translation (by the murabitun movement) this translation is usually seen as the most authentic.
For the ordinary reader however, the translation published by diwan press (of the murabitun) may be easier to read you may wish to have both copies to compare (as I have) Still, if you are reading this book for religious reasons alone, it should not matter which copy you happen to read.
North and West African Sufism has been largely overlooked in the West (possibly because of most of that geographical area coming under French influence) thus, we have little in the way of classical texts translated from that area, fortuantly that seems to be changing.
Other books I would recomend along side this one are:
fez city of Islam, The meaning of man (Sidi Ali jamal), The diwan of ibn al-Habib, The way of Abu Madyan, Zarruq the Sufi and the letters of ibn Abbad of Ronda.