The Armenian Genocide: Evidence From the German Foreign Office Archives, 1915-1916 edited by Wolfgang Gust for one of the single most compelling collections of promary sources you can hope for.

The Armenian Genocide: Cultural and Ethical Legacies edited by Richard G. Hovannisian for collection of essays addressing various aspects of the genocide and its legacy.

The Armenian Genocide: A Complete History by Raymond Kévorkian for a very thorough, in-depth history.

"They Can Live in the Desert but Nowhere Else": A History of the Armenian Genocide by Ronald Grigor Suny for a more basic, accessible read

America and the Armenian Genocide of 1915 by Jay Winter for internationalist angle.

Denial of Violence: Ottoman Past, Turkish Present and Collective Violence against the Armenians, 1789-2009 by Müge Fatma Göçek for Turkish-centric sources.

The Making of Modern Turkey: Nation and State in Eastern Anatolia, 1913-1950 by Uğur Ümit Üngör for placing it in the wider context of the period.

(With kudos to /u/yodatsracist for pointing me to those last two)