Http://escalade.camptocamp.com/ list a book for the Tarn published by the 'Club Alpin Causses et Cevennes'. Same area? Hope so coz Im also trying to research a trip over there soon. There is no direct link that I can see to order the book from their site but a search of the main French Climbing/Alpine Club sites might reveal who to order it from. Ive been putting off going that route as I dont speak french...better start learning, Alliance Francaise?!
There is no guide book of Gorges du Tarn on the net. But you can buy it 20ย everywhere (all the money made with this book go to buy new bolts)
If you are late in Millau look for the climbing gym "couleur Caillou" Rue de la prise d'eau. Open from 18 h to 22h you'll find there all informations for the Tarn.
Gorges du tarn