Two recently published books may be of interest. Visions of Annihilation: The Ustasha Regime and the Cultural Politics of Fascism, 1941–1945 by Rory Yeomans examines the internal dynamics of the movement and the relationship of its ideology to wider European politics. Robert McCormick's Croatia Under Ante Pavelic: America, the Ustase and Croatian Genocide examines some of the Cold War context of the Ustase movement.

Much of the history of Croatia in this period often gets enfolded into the larger history of Yugoslavia. Published with the assistance of his daughter, War and Revolution in Yugoslavia, 1941-1945: Occupation and Collaboration by the late Jozo Tomasevich is a companion to his early book on the Chetniks.

The December 2006 volume of Totalitarian Movements & Political Religions devoted the issue to the Independent State of Croatia and its articles are both insightful and their bibliographies should help in further reading.

There is also an interesting, if rather massive PDF, Pavelic Papers has various primary sources translated into English.

Hope that helps.