The Forever Knight by John Marco
"Lukien is the Bronze Knight, beloved by his kingdom and renowned in battle throughout his world. After betraying his king and losing his beloved, he wishes only for death, but rather than die, Lukien is given a chance for redemption: to be the protector of the Inhumans—those fragile mortals who live deep in the desert, far from the prying eyes of their world. These remarkable individuals have been granted magical powers in exchange for the hardships and handicaps life has handed them. And Lukien, now immortal himself, must be their champion. But how can one man, even an immortal warrior, protect hundreds from a world of potential enemies?"
Mass market, ex-library with usual markings