Is there a book which explains the reasons for the changes as to the feast days of saints?”
This is the document that makes and gives reasons for the changes. It is refferenced in articles about changes. for example St benedict-His feast day, previously 21 March, was moved in 1969 to 11 July, a date on which, in many areas, he was traditionally celebrated since the eighth century.: http://en.wikipedia.org/wiki/Benedict_of_Nursia St Thomas-“today celebrates his feast on January 28, the date of publication of the Summa. Before the revision of the Roman calendar in 1969 the feast was observed on March 7, the day of his death.”: http://en.wikipedia.org/wiki/Thomas_Aquinas another-“Calendarium Romanum (Libreria Editrice 1969), p.
