One cannot find anything more exhaustive or thorough on the history of Epeiros than N.G.L. Hammond's Epirus: The Geography, the Ancient Remains, the History and the Topography of Epirus and Adjacent Areas. 1967. Oxford University Press. 800+ pages. Anything else by Hammond you can find on jstor about Epeiros is going to be as accurate as anyone can be on it today.

Also see P. Cabanes' L'Epire de la mort de Pyrrhos a la conquete romaine. 1976.