The author was writing a book about spiritual Freemasonry and when the book pushed 500 pages the publisher asked to split it into three books because readers would be overwhelmed by a 500 page book. I personally would not have a problem and taken that this book is targeted at Freemasons (the publisher being Lewis Masonic) who, I guess, are used to reading too, I wonder if that was really the reason.
In any case, Spiritual Alchemy was published in August 2019. Then we have Freemasonry: The Quest For Immortality which was published in December 2019 and the upcoming Freemasonry: Initiation By Light (due April 2020). My guess was that Spiritual Alchemy was the first to read. Amazon has it listed as “spiritual Freemasonry series book 2” and towards the end I understand that the present title is mostly about the Fellow Craft degree (the second) and The Quest For Immortality about the first degree. Strange order of publishing! So when you buy the two available now or wait until all three books are available, I guess that you should start with The Quest For Immortality.