I recommend starting with the work of Oliver Statler.
The American scholar Oliver Statler (1915-2002) still stands at the center of Western studies on the Shikoku pilgrimage. If you want to learn more about the pilgrimage in English, the best place to start is almost certainly his book Japanese Pilgrimage.
From time to time I will meet fellow Americans who were part of the walking pilgrimages that Statler led around Shikoku. During his time the walking trails had been neglected for several decades, and it was a process of research and an adventure to simply find the right paths through the mountains. Now, kind red arrows and pointing hands show us the way!
-And don’t forget to support my Kickstarter East or West: A Walking Journey Along Shikoku’s 88 Temple Pilgrimage: http://kck.st/QB8XPQhttps://www.kickstarter.com/projects/1101472817/east-or-west-walking-a-japanese-buddhist-pilgrimag?ref=city