There is more than one way to Santiago de Compostella, making a network of routes through most of Europe. Wikipedia has your back on that one. Here’s a quick overview of the one we took: we followed the GR 653/Arles Way until Oloron-Sainte-Marie. Then we went off route in the mountains for a couple of days, and rejoined an variant of the GR 653, the GR 65.3.3 until Puente la Reina de Jaca.
From there, we followed the Aragonese Way (Camino Aragones) until Puente La Reina, and the French Way (Camino Frances) until the end. In France, we were mostly in forests and fields, with a few roads here and there. In spain, it was mostly fields and routes, and a few forests here and there.
We used mostly one guide, the famous “Miam miam dodo”, in two editions (one for Toulouse to Puente la Reina, one for the rest of the way). It’s a pretty good guide, with all you need to know for accomodations, maps, tourism offices, and more. I also want to thank the ACIR, a french association which helped us when we started.
You will also need a credencial, a kind of pilgrims passport. It will give access to pilgrims hostels and is necessary if you want your certificate of completion once you reach Santiago. You don’t not absolutely require it to walk, but it makes for a better experience.