In December 1976, in the lower right quarter of a page otherwise devoted to Tekumel miniatures, Dragon #4 squeezed in an early version of what would become Gygax's Appendix N in the Dungeon Masters Guide three years later. Perhaps this list was intended to provide material for the Christmas lists of readers? All of the authors in this version made it into the DMG list, save Algernon Blackwood, an early 20th-century writer of ghost/weird stories. Specific mention of Lord of Light by Zelazny was also dropped in the DMG. Otherwise, the 1976 list is shorter than the DMG list, missing Bellairs (Face in the Frost was reviewed by Gygax in Dragon #22 in Feb '79), Brown, de Camp solo, Derleth, Dunsany, Norton, Offutt, Pratt solo and Williamson.
Around this time Gygax was actively writing to a number of these authors. Tim Kask recently wrote on Dragonsfoot, in response to a question from myself about he came publish Fox's stories, that "Gary corresponded with several authors, including Gar Fox, Fritz Leiber, L Sprague De Camp and Lin Carter, to name just a few. As publisher, he gave me an intro and the rest fell into place. My biggest thrill from that was reprinting Fritz's story [Sea Magic, Dragon #11], closely followed by the original Niall stuff [Gardner Fox, 10 stories between 1976-1981] and a De Camp story [a reprint of the Harold Shea tale The Green Magician, Dragon #15-16]. Here I was, a nobody at a new weird game company, actually publishing writers whose works I had been reading for years. Heady times". Leiber and Fox were also both guests at Gen Con '77.
During this time period Gygax was also corresponding with Dr. Holmes, which led to his work editing the Basic Set. The exact chronology is murky but presumably work on the Basic Set was well in progress by late 1976 as it was published during the middle of the following year. Dr. Holmes was a fan of many of the same authors as Gygax, which I can only assume facilitated their collaboration. 1976 saw the publication of Holmes' first novel, The Mahars of Pellucidar, an authorized sequel to Burroughs' inner earth series. On page 41 of the Blue Book, Holmes included a proto-Appendix N, writing "The imaginary universe of Dungeons & Dragons obviously lies not too far from the Middle Earth of J.R.R. Tolkien's great Lord of the Rings trilogy. The D & D universe also impinges on the fantasy worlds of Fritz Leiber, Robert E. Howard, Gardner F. Fox, classical mythology and any other source of inspiration the Dungeon Master wants to use".