The route is described in detail with good maps in: Last modified February 2009
GR20: Books
GR20 - réf. 067 - A travers la montagne corse - Parc naturel régional de Corse - Calenzana / Vizzavona / Conca
This guide is written in French, but that may not be a problem. The route is waymarked extremely well and the maps in the guides give enough information if it is not clear where the route is going. In the booklet estimations of walking times are given, which were good when climbing and most of the time too short when decending - especially for me.
Next to the official French guide, we also bought an English booklet from Cicerone. This booklet describes the route well and gives a short summary of each etappe, but has poor maps. Therefore during the walk we did not carry it with us.