This is not a novel, but "longish" short fiction, novelette or novella. I read it around 1984, in an already used book containing half a dozen or so stories, in a cheap hotel in the US. There were several such books on a shelf, there. I also read another, very similar one, that contained "Nightwings" by Robert Silverberg. After checking all collections containing the latter published before 1985 (thanks, Jenayah !!!), I got to the conclusion those were two different books. It is not a bad story, but it did pale compared to "Nightwings" ! A man, on our Earth, is contacted by a bizarre agency who offers him to get his mind transferred to a body in an alternate Earth, in order to save, either some polity from an external enemy, or the queen of some polity threatened by an "internal" cabal, I forgot. Why he accepts is not clear, since when will he be able to use the money he is offered? No return seems promised. He gets there, and meets several speci...