This is a double novel, with Robert Silverberg's "Born With the Dead" on one side and "Brian W. Aldiss's "The Saliva Tree" on the other. Front of the Silverberg side says: "The dead walk...but only with each other." Front of the Aldiss work says: The aliens were hungry---and Earth was perfect farmland!"
The two books in one both start from one side and go to the middle, then you turn the book over and read the other one!! (Like the old "Ace Double Novels.")
Silverberg's "Born With The Dead" is great. A look at the futility of obsessive, unrequitted love.
The Aldiss novella is a fun page turner. It's more of a sci/fi horror novel. Well written. And it has some excellent surprises, which I don't want to spoil for you. But it's not one of his 'serious message' works. It's just a quick fun, page turner.