I am trying to remember a novel that I read some of. I think I got it from my uncle's collection, which would mean it was published before 1975, and I was reading it before 1990. I don't remember that much of the plot, except that it involved a primitive tribe of humans (who seemed to be living on a post-apocalyptic Earth), who were visited by what were clearly identifiable (to the reader) as humanoid aliens, who arrive and begin kidnapping people. A human warrior stows away in the aliens' travel device to try to figure out what is going on and to rescue the kidnapped people.
That was pretty early in the book, I think, but it's where I stopped reading. I stopped because I had only been reading the book because I thought somebody had recommended it to me; however, that person had actually meant a different SF book with a similar name. In fact, I think the name of the book I am thinking of was probably deliberately confusing, like "mockbuster" movies.