You can download Rabbi Bernard Revel זצ''ל's doctoral thesis The Karaite Halakah and its Relation to Sadducean, Samaritan and Philonian Halakah, Philadelphia, 1913.
I haven't read it yet, but here is an interesting excerpt from pg.2
The question of the origin of Karaism, its causes and early development is still awaiting solution. That Karaism is not the result of Anan's desire to revenge himself on Babylonian official Jewry, need not be said. Karaite literature affords us not data; there is a marked lack of historical sense among them. They have no tradition as to their origin, and their opinions are conflicting. The belief that Karaism is but an echo of a similar movement during this period in the Islamic world is now generally given up owing to the advance made in the knowledge of the inner development of Islam, and, particularly, the nature of the Shiite heterodoxy.