The How and Why of Tolkien's historical and moral framing of his work

This question has fascinated for more than forty years since I noticed that the Prologue and several Appendices to The Lord of the Rings adopted the 'conceit' that the book was derived from an historical text called The Red Book of Westmarch and Tolkien was its translater and adaptor.

This topic was vital to Tolkien, and the matter is therefore well worth pondering, as I do at the Notion Club Papers blog.

(The unfinished novel, The Notion Club Papers, is itself a major attempt at framing the on-going writing of The Lord of the Rings.)