Among the bibliography on one of the pages on Aght'amar, I saw the list below. I own the Mnatsakanian Volume, and it's nice. I didn't know the RAA Milan edition existed, but have an idea it will look pretty much like the other RAA volumes.
I am curious about the contents of the Davis volume though, regularly see it online selling fairly expensive but have not seen any reviews of it. Does it have better photos or written content?
DAA = Sirarpie Der Nersessian and H. Vahramian, Documents of Armenian Architecture, Volume 8, Aght'amar, Milan, 1974.
Davis = J. G. Davis, Medieval Armenian Art and Architecture: The Church of the Holy Cross, Aght'amar, London, 1991.
Mnatsakanian = Stepan Mnatsakanian, Aghtamar, The Church of the Holy Cross, New Jersey, 1986.
Nersessian = Sirarpie Der Nersessian, Aght'amar, Church of the Holy Cross, Cambridge, Mass., 1964.