My distaste for the harm L. Sprague De Camp wrought to the reputations of Robert E. Howard and H.P. Lovecraft notwithstanding, there's no denying that he was an important figure in the early days of fantasy and science fiction and that his writings greatly influenced Dungeons & Dragons. De Camp is almost unique among the writers Gary Gygax listed in Appendix N in that his name appears not once but twice -- once alone and a second time linked to that of his writing partner Fletcher Pratt, with whom he co-wrote the Harold Shea stories. (Pratt, of course, is the only other author whose name appears twice)
Consequently, it'd be a grave mistake to overlook the De Camp books listed in the back of the Dungeon Masters Guide. Among them is 1941's Lest Darkness Fall, originally published in 1939 as a short story in Unknown magazine. Like many of the books that influenced D&D, Lest Darkness Fall is the story of a modern person transported to another world -- in this case, 6th century Italy -- who, through a combination of modern know-how and determination, changes this other world forever.
The modern person of De Camp's tale is Martin Padway, an American archeologist, who, after using his knowledge to introduce unheard of inventions (brandy, printing press, newspapers, telegraph) and thereby make himself wealthy, becomes involved in the politics of Ostrogothic Italy. Over the course of the novel, Padway's actions change history by stabilizing the Ostrogothic kingdom, beating back the Byzantines, and even preventing the foundation of Islam. What's interesting is that Padway never once considers the consequences of his actions in the past, even though they will undoubtedly destroy the future from which he came. I confess that De Camp tells the story with such good humor and verve that I didn't much care about such things, preferring instead to enjoy the novel for what it is: an amusing tale well-told. Harry Turtledove, contemporary master of alternate history, agrees and credits Lest Darkness Fall for igniting his interest in history and, by extension, alternate history.
Alternate histories and parallel worlds are a significant part of the pulp fantasy heritage of Dungeons & Dragons. They've largely been lost in the game's modern incarnations. Indeed, I can't recall any specific connection between a D&D world and our own in anything published in over 20 years. Once upon a time, that wasn't the norm: adventurers regularly encountered dimensional castaways from 20th century Earth or raided the British Museum in search of the Mace of St. Cuthbert. I miss things more than I realized, which is why I plan to do my part to restore this aspect of D&D's heritage in my own campaigns.