A friend lent me a book awhile back, and my memory is sketchy on the plot, but I would like to find this book again.  What I recall is that the protagonist goes to a planet of ghosts. . .at some point the protagonist goes to an underworld of the planet--maybe this is where the ghosts come in?--and is not supposed to be able to return from there, but does.  What I recall of the underworld is very sketchy, too, but he finds himself in a dark icky place, held in a dungeon-like place by the drones of the underworld.  Eventually, he is able to get out of their grasp and meets the ruler (or whatever) of the underworld, and then is able to get back to the place he came from.  Since my memory of this book is so sketchy, not sure this will ring any bells.  What I am describing is only a small portion of the plot--and unfortunately, was the part of the book I didn't enjoy very much which is probably why it stuck.  It really was a good read even though I don't do the book justice here.