The Northern Songhay family is of some general interest, both for the study of language contact - all its members are astonishingly strongly influenced by Berber and/or Arabic, to the point that only a few hundred Songhay words survive and much of the grammar has been replaced - and for understanding the history of the Sahara (they suggest both that the spread of Songhay predates the Songhay Empire and that a Berber language different from Tuareg used to be spoken in much of Mali and Niger.) I've recently put together a sort of homepage for Northern Songhay linguistics: Northern Songhay. It includes a more or less complete bibliography.
Anyone interested in that will also be interested in a site I recently came across: Songhay.org, offering lexicographical data, lessons, software, and some references focused mainly on the Songhay of Gao (Koyraboro Senni.) I particularly appreciated the pictorial dictionaries under "Encyclopédie".