The first Horseclans novel,The Coming of the Horseclans by Robert Adams, was early enough that it barely misses pre-dating the original Dungeons and Dragons. While the author, Robert Adams, would average a book a year in the seventies the series really hit its heyday in the early eighties. I wouldn't be surprised to find out that many of those fans were D&D players looking for something to inspire or feel like playing their favorite game.
The world of the Horseclans is a post-holocaust United States ranging from later this century (World War III having happened in 1980) to the cusp of the third millenium. In this future humanity has two races: normal humans and mutants whose aging stops in roughly their thirties and who are immune to most wounds. At the beginning of The Coming of the Horseclans Milo Morai, one of these mutants, is leading a migration of the Horseclans from the Sea of Grass to their mythical home city of Ehlee on the east coast of North America. In the aftermath of the war, Milo rescued a group of children who will become the ancestors of the Horseclans and instructs them in the ways of horse nomands before leaving them to search for a mythical land inhabited by people like him. Accompanying them are their allies the Horse clan (horses are free citizens of the Horseclans, not owned beasts) and the Cat clan of sabertooth tiger like praire cats. The Horseclans communicate with their animal allies and each other by a ubiquetous telepathy.
In the the centuries since the war the world has changed both due to natural diasters and human migrations. The most dramatic of these migrations revealed in this book is the Ehleen invasions of North America. The Ehleens are Greek speaking people from Europe. Dwelling along in several kingdoms along the Atlantic seaboard they are the principle opposition to the migration of the Horse Clans. Coming of the Horseclans plot centers on the conquest of Kehnooryos Ehlas, until a few years before the novel the most powerful of these kingdoms. Several battles, raids, and duels make up the novel. Finally, one of the more interesting aspects of the Horseclans' world, the mysterious witchmen, make a brief appearance.
If I had to select one word to describe this book it is violent. R. E. Howard and Jack London's belief that civilization is a corrupting influence is writ large. While the Horseclans keep slaves and look down on the dirtman farmers they conquer they maintain a strict code of honor and morality that disdains killing and lying. However, the corruption of civilization goes very deep and many of the Ehleens are pedophiles among other perversions. In fact, the sexual aspects, especially of the depraved characters, are not completely glossed over and the reader should have fair warning that sexual multilations are described with some detail in a few sections. The book also portrays homosexuality as a corruption of civilization. It is certainly openly offensive to modern sensibilities. It was, even in its time, a bit of a throw back to early 20th century in morality and ideals. It added in the graphic description available to a 70s author.
Adams would go on to write seventeen more Horseclans novels over the next thirteen years as well as two shared worlds anthologies with fans. The first book, however, was no clearly the first of a series. Later copies, in fact, added two short preludes to set up Milo's return to the Horseclans that were not in the original printing. Given it's age and content it might be surprising that I consider it appropriate for Silver Age Appendix N. There are three reasons I do. First, the series it spawned would hit its height of popularity about the time I graduated from high school in 1985 and stay there through Adam's death in 1990. It first gets reviewed and discussed in The Dragon in the mid-eighties. Second, it would be one of the first GURPS source books and the first of their very successful run of one shoot licensed source books. Finally, for a men's action series it has an unusually large female fan base. The best theory I have heard on this is many women bought the fourth book, A Cat of a Silvery Hue, based on the title.
Until recently Coming of the Horseclans has been out of print, but in the past couple of years Mundania Press brought the first book back into print. It used to be easily found at used bookstore, as well, but in the past five or so years it seems to have gotten rare. If you want some "new" swords and sorcery fiction to inspire your game I can't recommend it enough.