Asura in Early Vedic Religion
by Wash Edward Hale

In the Rg-Veda the word asura is frequently used as an epithet for various deities, however, in later Sanskrit it is used instead for a class of demons who are enemies of the gods. 

In this book the Vedic passages containing the word asura - are examined to see how such a remarkable shift in meaning could occur.  Every verse in the Samhitas containing the word asura - or a derivative or a compound of it is quoted, translated, and discussed.