Robert Smith 
Kingdoms of the Yoruba [PDF ebook] 

Originally published in 1969 and as a second edition in 1976, this book gives a general account of the major Yoruba kingdoms and provides a synthesis of Yoruba and Igbomina history, culture and archaeology. The reasons for, and the chronology of the decline and fall of Old Oyo are also discussed. Much of the history reconstructed in this book was done so almost wholly from oral histories, with all evidence being subjected to rigorous examination.