As for why he wrote a second book (it’s sort of a sequel to Heretics), he explains why in the preface:
This book is meant to be a companion to Heretics, and to put the positive side in addition to the negative. Many critics complained of the book called Heretics because it merely criticised current philosophies without offering any alternative philosophy. This book is an attempt to answer the challenge. It is unavoidably affirmative and therefore unavoidably autobiographical…. It is the purpose of the writer to attempt an explanation, not of whether the Christian Faith can be believed, but of how he personally has come to believe it. The book is therefore arranged upon the positive principle of a riddle and its answer. It deals first with all the writer’s own solitary and sincere speculations and then with all the startling style in which they were all suddenly satisfied by the Christian Theology.
As usual, it’s available to download in EPUB or Kindle formats.