The Unknown Craftsman by Soetsu Yanagi — not specifically about Nancy, but essential reading for anyone interested in the Arts and Crafts movements that gave rise to the École de Nancy. Yanagi's meditation on the beauty of handmade objects, written from a Japanese perspective, illuminates what Gallé and his colleagues were doing and why it mattered.
A Moveable Feast by Ernest Hemingway — though Paris-set, this memoir of the writer's apprenticeship years mentions his travels through Lorraine and Alsace. Good companion reading for a trip through eastern France, and it gives context to the France of the early 20th century when the École de Nancy was at its height.
The Tin Drum by Günter Grass — set mostly in Danzig but ranging across the Franco-German borderlands of the Second World War. Lorraine is the contested zone at the edge of the story, and the novel gives a visceral sense of what this part of Europe meant in the 20th century. Grass won the Nobel Prize partly for this book.