The easiest way to book rail tickets is through the SNCF website, where you are given the choice of an eticket (to print at home or to save on your mobile), or a hard copy ticket sent to you by mail. That way you can step straight on to the train without queuing up at the ticket counter.
Fédération Française de la Randonnée Pédestre (FFRP)
A full list of available topo-guides can be seen and bought on the FFRP website.
Institut Géographique National (IGN)
All the TOP 100 and TOP 25 maps, and the topo-guides, can be bought on the IGN website.
Géoportail is the French equivalent of Google Maps, but with far greater detail. This is very useful when designing your own route.
Map showing all GRs and GRPs
It is surprisingly difficult to find a map showing all the GRs and GRPs. The only such map that we know of can be seen and downloaded via this link to the FFRP website (it is a big pdf file and can be slow to load).
GR-Infos is a useful site for general information about GRs.
My long walk
My long walk contains interesting diaries by an American who has walked extensively in Britain, but has also done two long walks in the south of France.