You should be able to find a guidebook to the GR11-- here's one. Google is your friend here. All of these famous long trails have detailed guidebooks. I have a guide to the hiking trails of Corsica and another to the "Chemin de Stevenson," the trail through the Cevennes following the route taken by Robert Louis Stevenson (of "Treasure Island" fame). Both of these are in French, though, since I bought them "on location."

It wouldn't hurt to brush up on your French (for the GR 10) or Spanish (for the GR11). If nothing else, you'll get a lot more out of your journey if you can communicate with the people you meet!