I have a few more suggestions:
By far the best book on Bulgaria I have read would have to be "Domesticating Revolution: From Socialist Reform to Ambivalent Transition in a Bulgarian Village" by Gerald W. Creed, he has a new book out titled "Masquerade and Postsocialism: Ritual and Cultural Dispossession in Bulgaria" which sounds equally as interesting.
"The Balkans: A Short History" by Mark Mazower is a good quick history book on the Balkans.
For the military reader "Balkan Breakthrough: The Battle of Dobro Pole 1918" by Richard C. Hall is well-researched and informative, but badly written. It just came out in 2010.
"Roma in an Expanding Europe: Breaking the Poverty Cycle" by Dena Ringold, Mitchell A. Orenstein, E. Wilkens, Erika Wilkens (aka the World Bank) is interesting if you are interested in the Roma population of Bulgaria.
"The Miss Stone Affair" by Teresa Carpenter is another great book about Yane Sandanski and his kidnapping of an American missionary to further the cause of the BMRO.
Last suggestion, "Voices from the Gulag: Life and Death in Communist Bulgaria" by Tzvetan Todorov. Intense and interesting.