Rayner Unwin had nearly the most singularly important role in getting "The Lord of the Rings" by J.R.R. Tolkien published.
As a young boy, it was his enthusiasm for the story of "The Hobbit" that played in influencing his father - Stanley Unwin of Unwin & Allen - to publish that volume. Rayner's own clamouring for more - a sequel - as well as other fan letters asking for the same that prompted Stanley to frequently inquire to Tolkien for another book (something he had never intended to do).
As the "Lord of the Rings" was started, it was Rayner - as well as the author's son, Christopher Tolkien - that received early copies of the chapters to serve as editor and reviewer. Rayner continued to indicate, enthusiastically, that it was worthy.
World War II of course interrupted the text in many ways, and Rayner grew up, but he never lost interest. Tolkien drifted away from using the publisher Unwin & Allen (due in some small part to stubborness on Tolkien's part). And yet, the project was further stalled when the other publisher he courted failed to take it print either. It was Rayner - come into his own in the family business - who again approached Tolkien in early 1952 about publishing LoTR.
As an interesting side note on Rayner's passion for the book, he ended up naming his first son Merlin, and even apparently felt it necessary to mention to Tolkien why he thought that name better than Gandalf.
Without Rayner Unwin one of the great works of English Literature may have wallowed, unpublished for a long time indeed.
Much of this is documented in the the fascinating book "The Letters of J.R.R. Tolkien" by Humphrey Carpenter.