Start with Mervyn Hiskett, *Sword of Truth*. A good shorter
introduction is Murray Last's chapter in Ajayi and Crowder, *History
of West Africa*, Vol. II. Last and Barkindo have chapters in Volume I
that set the scene nicely. David Robinson and Douglas Smith, *Sources
of the African Past*, have a collection of source readings dealing
with Usman. The most interesting, a debate on jihad conducted by his
son, Muhammed Bello, is also excerpted in Thomas Hodgkin, *Nigerians
Perspectives*.