There’re numerous books on medieval pilgrimage, and even though I don’t agree with all of Sumption’s conclusions, I’m recommending this for its readability and fascinating anecdotes and quotations, drawn from contemporary accounts, which were invaluable for my research. It’s informative about both the devout and more worldly travelers, kings, queens, clerics and nobles, and the common people of the day.
One major drawback is that his focus is largely on France and Rome, while Jerusalem, Santiago, and the German pilgrimage sites don’t get the attention they deserve. But this just demonstrates how popular pilgrimage was throughout the Middle Ages and how busy the highways and byways were with crowds of pilgrims. I learnt that people were much more mobile at this time than I had thought.