The Church of Armenia
by Patriarch Malachia Ormanian is something of a classic. You can find it through St. Vartan Bookstore: http://www.stvartanbookstore.com/index.asp
I don't think it is listed on their website, but they have it. Ask them about it in an e-mail.
The only problem with that book is that it was written prior to the Genocide. I'm not sure what books there are available in English telling what happened to the Armenian Church during and after the Genocide. Ask the bookstore about that and I am sure they will let you know.