Maps showing the hiking trails and cross-country ski trails are available in even the tiniest book and news shops.
The blue 1:25'000 maps by the IGN (Institut Geographique National) are extremely detailed, and the best for hiking. Hiking trails are shown along with topographical lines, altitudes, and other useful landmarks. The downside is, if you're hiking long-distance or in several different locations, you'll need a lot of thes maps, and they're fairly expensive.
IGN maps are available for purchasing on the IGN website (www.ign.fr), but the site has become very complicated and confusing (although no doubt with a lot more features, whatever they may be [autumn 2016]. The Top Serie Bleue maps (1:25,000) are shown on IGN web page [Top25 Series Blue].
Hiking Mobile App
IGN, IphiGéNie Hiking
This is an online version of the famous French IGN (1:20'000) hiking maps, but includes other scales as well. The most detailed scale (about 1:1500) gives a satellite-image view.
If you're going to be hiking for more than a week or two, or doing serious hiking in or across France, we strongly recommend this IphiGéNie app. (An unfortunately "cute" name, we just call it the IGN app).
The app is free, of course, but the complete maps of France cost about 15 euros for a year's subscription. We've been using it for a few years now.
Map segments are downloaded dynamically, at whatever scale you want. The downloaded map segments remain on your mobile device, for use out in the wilderness where there's no internet and no cellphone coverage. Before setting out on a hike in a remote area, you download the maps for that area from the comfort of your home (or hotel) wifi,
The IGN map app displays the map, of course, with finger swipes/pinches to change the scale dynamically, and your current position. You can put on a trace that follows you on your hike (and can be named saved), drop pins (that can be named and saved). The trace displays the elapsed time, distance, altitude change and average speed.
The map image at the top of this page is a screenshot of a segment of a hike passing the Pont du Gard; our route trace is pale blue.