Prairie Caesar is the seventh book in the Six-Gun Samurai novel series, re-branded with this entry as the Six-Gun Warrior series. Not quite sure why they changed the name, but considering that this cover features the main character in typical cowboy getup instead of the traditional Japanese garb on previous covers, it probably had something to do with an attempt to pick up sales by making it look more like a typical western novel.
The main character of the series is Tanaka Tom Fletcher, born in America but raised in Feudal Japan as a samurai warrior, only to return to the states to exact vengeance on Colonel Edward Hollister and his regiment for the murder of his family. Prairie Caesar finds him traveling to Nebraska city to hunt down one of the men on his death list, Carlton Gray. Along the way, Tanaka Tom comes to the aid of settlers in need, sleeps with almost every woman he comes in contact with, almost dies from pneumonia, and makes friends with Sioux and Cheyenne tribes. Oh, and he also manages to get himself kidnapped by centurion soldiers and imprisoned as a slave in an ancient Roman city in the desert run by a direct descendant of Caligula.
Geographically hidden from modern society, this miniature incarnation of Rome is building its forces for an attack on the United States, as its deranged and decadent leader is obsessed with world domination. Tanaka Tom's performance in the forced gladiator battles catches the eye of a treasonous faction looking to overthrow Caligula, embroiling him in the political intrigue even as he attempts to plan his escape and help his fellow slaves.
Western novels probably don't get weirder than this.