The Pilgrimage Road to Santiago by David Gitlitz and Linda Davidson — A serious scholarly work on the great pilgrimage routes of medieval Europe, covering the Via Lemovicensis that passed through Vézelay. More history than travel writing, but it gives excellent context for what you are looking at.
Romanesque Art by Henri Focillon — The classic account of Romanesque architecture and sculpture, written by the great French art historian. Focillon's analysis of how Romanesque carvers worked — their relationship between stone and figure — helps you understand why the Vézelay tympanum is as remarkable as it is.
The Cathedral Builders by Jean Gimpel — An accessible and enthusiastic account of how the great medieval churches of France were actually built: who did the work, how it was financed, what tools they used. Good preparation for a visit to Vézelay or any of the Burgundian Romanesque churches.