Hiero's Journey by Sterling Lanier was published in 1973 and is unique for having been an influence not just on Dungeons & Dragons but also on Gamma World. The novel chronicles the adventures of the eponymous Hiero Desteen, telepathic priest and "killman" of a futuristic descendant of the Catholic Church, as he searches for lost technology to use against the Dark Brotherhood, would-be conquerors of the postapocalyptic world they all inhabit. The connection to Gamma World is obvious to anyone who ever played the game, but I suspect the D&D connection is less clear and perhaps understandably so. Nonetheless, I continue to hold to the opinion that pulp fantasy frequently possesses strong postapocalyptic overtones, with the action taking place in the aftermath of the collapse of some mythical Golden Age. Exploring and looting "dungeons" certainly makes more sense in this context, as does the lawlessness of the implied D&D setting. It's yet another reason why I find high/epic fantasy a poor fit for the game.
I was very fond of Hiero's Journey as a kid. What's not to like about a psychic warrior-priest with a mutant moose and bear as companions? There was sequel to the book -- The Unforsaken Hiero -- which ended on a cliffhanger, as I recall. There was never a third book in the series, which may be just as well. Third books (or movies) are frequently the weakest offerings in a series and I'm glad my fondness for these characters isn't sullied by knowing the conclusion didn't live up to my expectations.