The John Brierley book has adequate maps for the Camino Frances. The path is well marked with yellow arrows, so you are not likely to get lost without a map. But maps are handy for evaluating alternatives.
That's true. However, if you are like me and just like maps for the pleasure of pouring over them, judging how far you have come and still have to go, whether the hill you can see ahead is one you are going to have to climb etc, then you can get maps easier in France than in Spain, I think. We are cycling our camino, so we buy the French IGN Carte de promenade (1:100,000). If you are following arrows as well, these might be good enough for a walker, but the footpaths are thin single black lines. There are 1:50,000 available as well. We found them in shops as we went, especially in places like Vezelay, so I expect they will be some in Le Puy. About 5 euros, I think. Cheaper to buy in France than by mail order. But I believe Stamfords is the place to order from in the UK, anyway.
It seems to be harder to find proper maps in shops in Spain, but I have less experience of that. I ordered the maps for the Camino Ingles from http://www.tiendaverde.es .I think 3 plus postage came to about 20 euros.
Gulliver, seeing that this post appears in the Le puy section of the forum I am assuming that you are interested in that part of the path. There are maps available for that section, but I would suggest that purchasing the relevant Topo guides (there are 3 for the Le Puy route) would be a good idea. They are in French, but on each facing page the relevant map (I think it may be a scale of 1:50thou. but not sure - the detailed scale at any rate) appears opposite the descriptions of the path. From memory the guide books are from le Puy to Figeac, Figeac to Moissac and the last one from Moissac to the Pyrenees. I don't have copies of them but is roughly how they are divided. The maps in these guides are very detailed showing exactly where the path goes, but also be assured that the path is very well waymarked.