The Underdogs by Mariano Azuela (1915) is the classic novel of the Mexican Revolution, written by a Jalisco doctor who lived through the conflict. It gives essential background to the violence that tore through Michoacan and the rest of Mexico in the early 20th century.
Like Water for Chocolate by Laura Esquivel — though set in northern Mexico, this magical realist novel explores the food, family, and culture of Mexican domestic life in ways that apply across regions. Michoacan's own rich food traditions are a perfect companion subject.
The Power and the Glory by Graham Greene is set in the state of Tabasco during the Cristero War period — the anti-Catholic persecution that hit Michoacan particularly hard in the 1920s and 1930s. Reading it gives context to the religious intensity still visible in Morelia's many churches.
Monarch Butterflies: Their Journey by various natural history writers — there are several good popular science books on the monarch migration. Understanding the biology and the threats to the migration makes the biosphere reserve visit far more affecting.