The Mystery of Olivier Bécaille by Émile Zola. A short story set partly in 19th-century Montpellier; Zola passed through several times and his eye for southern French detail is sharp.
The Labyrinth by Kate Mosse. A bestselling novel that weaves between modern-day Carcassonne and the Cathar wars of the 13th century. Languedoc is the real subject — read it before driving the Cathar country south of Montpellier.
A Tale of Two Cities by Charles Dickens. Not Montpellier itself, but Dickens spent time in the Languedoc and the south of France shapes much of his French settings. Less direct but still useful: Stevenson's Travels with a Donkey in the Cévennes, which describes the rugged country an hour north of Montpellier.
Languedoc-Roussillon: The Wines and Winemakers by Paul Strang. A serious working reference for anyone wanting to understand the vineyards around the city. Read this and you will drink better for the rest of your trip.