Here is an article that I wrote this year for Visit Kagawa: Kagawa Official Tourism Website. It is about Zentsuji and the birthplace of Kukai. It published this year, but I hope it will always be relevant to people looking to explore the spiritual roots of the Shikoku Pilgrimage in Japan.
Japanese Temple, must see kagawa, Shikoku, Your Pilgrimage in Japan
Spiritual Birthplace of Shikoku
Birthplace of Kukai