[Cathedral of Santiago de Compostela (Church of St. James), Obradoio facade, 2007, photographed by jonathan jacobi, source: Wikimedia Commons]
Sometime in the late 9th century, Alfonso III of Asturias began promoting the shrine as a pilgrimage destination, rebuilding the church there and talking to French bishops about it. This may have prompted the first French pilgrims to make their ways there. By the late 10th century, the shrine was attracting people from as far away as Armenia and had become the third most important pilgrimage destination for Europeans after Jerusalem and Rome.
[Map of the Camino de Santiago in Europe, Manfred Zentgraf, source: Wikimedia Commons]
[Cathedral of Santiago de Compostela from the Tribuna, 2009, photographed by Lansbricae, source: Wikimedia Commons]
*There are many stories as to how this is supposed to have happened.
Webb, Diana. Medieval European Pilgrimage. New York: Palgrave, 2002.
Barber, Malcolm. The Two Cities: Medieval Europe 1050-1320. London: Routledge, 1993.
Search Google or Google Books and you'll find dozens of guidebooks and reflections on people's experiences.
The Way is a movie about an American eye doctor walking the Camino. I found it rather uninspired but a lot of people seem to like it.