The Huizhou Merchants in Ming and Qing China by Harriet Zurndorfer — An academic study of the merchant families whose wealth built the villages of Wuyuan and the broader Huizhou region. Dry in places but authoritative on the economic basis of the architectural splendor you'll see in the villages. Available in academic libraries.
Culture and Commerce: The Tea Industry in Late Imperial China by Robert Gardella — Tea was one of the pillars of Huizhou merchant wealth, and Wuyuan's tea culture connects directly to the commercial networks this book describes. Provides useful background for understanding why such elaborate architecture exists in these remote valleys.
Dream of the Red Chamber (红楼梦) by Cao Xueqin — Not set in Wuyuan, but the social world of the Qing dynasty gentry-merchant class that this novel depicts — the elaborate household architecture, the emphasis on feminine virtue and male scholarship, the anxiety about family decline — is directly relevant to the culture you're seeing preserved in these villages.