Children of the Gates by Andre Norton (Paperback, 2013)
On a world where parallel and alternate universes intersect, Celtic faery folk, modern humans, and inhabitants of a distant future share the planet surface. There are also aliens in flying saucers which are capturing and enslaving new arrivals who fall through the gates. One group of new arrivals meets a protector, a man who calls himself the Herald. He promises safety - but in exchange, he requires a piece of their souls in payment. Children of the Gates features two powerful science fiction adventure novels (Here Abide Monsters and Yurth Burden) in one trade paperback volume from master storytelling legend, Andre Norton.