Reading about Burundi before you go is not just useful -- it is almost necessary. The country's history is complex and often heartbreaking, and understanding even the basics will transform what you see on the ground.
Burundi: Ethnic Conflict and Genocide by Rene Lemarchand is the essential academic text on Burundi's troubled history. Lemarchand, who spent decades studying the Great Lakes region, traces the roots of Hutu-Tutsi conflict with more nuance than you will find anywhere else. Dense but rewarding if you want to understand why Burundi is the way it is.
Purity and Exile: Violence, Memory, and National Cosmology among Hutu Refugees in Tanzania by Liisa Malkki is a landmark anthropological study of Burundian Hutu refugees in Tanzanian camps. It is an academic work, but beautifully written and deeply humane. Malkki explores how displaced people construct meaning and identity -- themes that resonate far beyond Burundi.
Historical Dictionary of Burundi by Ellen K. Eggers and Warren Weinstein is a reference work rather than a narrative, but invaluable for looking up people, places, events, and concepts. Useful to have on hand as you encounter things you want to understand better.
Genocide: Rwanda and Burundi by Edward L. Nyankanzi covers the parallel histories of violence in both countries. While the Rwandan genocide gets far more international attention, Burundi's own cycles of mass killing are equally important to understand. Nyankanzi provides a regional perspective that many single-country studies miss.