Zapotec Science: Farming and Food in the Indian Villages of Oaxaca by Roberto González Villareal — an anthropological study of traditional Zapotec agriculture and food systems that gives a deep grounding in how the land and culture of the valley relate to each other. More readable than it sounds.
In the Shadow of Tlaloc: Life in a Mexican Village by Leigh Binford — a close study of a Mixtec community in the Oaxacan sierra, covering land, labour, and daily life. Good preparation for understanding the living indigenous culture you will encounter in the valley villages.
Like Water for Chocolate by Laura Esquivel — set across the border in northern Mexico but the most evocative literary treatment of Mexican food culture. Read it before you go; you will taste the cooking differently.
The Lacuna by Barbara Kingsolver — a novel that moves between Mexico (with Frida Kahlo and Diego Rivera as characters) and Virginia; the Mexican sections are vivid and historically grounded, though Oaxaca itself barely features.