Books about pilgrimage walks come in two kinds, and most people need one of each. The first is a practical guide: stage-by-stage route descriptions, maps, places to sleep and how to get home again. The second is a writer's account of the walk, which is no use for finding your way but a good deal more use for deciding whether to go.
This list takes both. The practical guides cover the St Francis Way in Italy, the Camino Francés across northern Spain, the Camino Ignaciano in the Basque Country and Catalonia, and the Fife Pilgrim Way in Scotland. The narratives are mostly about the Camino de Santiago, with two recent books on pilgrim paths in Britain. For Italy's other great pilgrimage, see our page on the Via Francigena.
Check the edition before you buy a guidebook. Routes are re-waymarked, stages are re-cut and accommodation changes, so a guide a few editions old can describe a different walk from the one on the ground. The current St Francis guide, for instance, describes 518 km in 27 stages, where the first edition described about 550 km in 28.