"The absence of answers or determinate meanings" is exactly the set of "qualities that make a passage or a work literary." Literary works have no single meaning, whatever the author intended. Indeed, Garber points out, "one of the key features of what might be called the literary unconscious is a tendency on the part of the text to outwit or to confound the activity of closing or ending."
This is not relativism. This is not deconstruction. People have been saying things like this about literature since Horace - and indeed, Horace's ancient dictum (that literature should both teach and delight) is, I believe, one of the underlying themes of Garber's book. It's just that what Garber thinks literature teaches is not a set of univocal moral truths but rather a habit of mind: a way of questioning the world, a way of understanding just how hard it is to make decisions, fall in love, express desire, worship, rule and serve.
We read books often to learn how others do these things - and often to learn how others failed to do them. We read books to be pleasured, too, into an admiration for a writer's choice of words or for an author's command of our emotions.