The Baltic Revolution: Estonia, Latvia, Lithuania and the Path to Independence by Anatol Lieven (1993) — Still the best single-volume account of how the three Baltic states broke free of the Soviet Union. Scholarly but readable, and essential context for understanding modern Latvia.
The Latvians: A Short History by Andrejs Plakans (1995) — A compact and reliable history from ancient times through independence. Plakans is a Latvian-American historian and writes with care about a complicated national story.
Walking Since Daybreak: A Story of Eastern Europe, World War II, and the Heart of Our Century by Modris Eksteins (1999) — A mixture of history, memoir, and travelogue that traces the author's family's displacement from Latvia during the war. Beautifully written and gives a real sense of what Latvia lost in the 20th century.
Lonely Planet Estonia, Latvia and Lithuania — The standard practical guide for the region, updated regularly. Useful for logistics even if the prose is workmanlike.