Adding a shine to a book cover

If it's going to be an ACTUAL book cover, they will probably use a varnish or shiny card or special ink for this effect, so you don't try to allow for it in the design. If it's a home printed book cover, or a picture of a book, it's a different story.

If realistic light and dark areas are what you seek,  I would start with a Gradient Map Adjustment Layer. 

Gradient Map Adjustment Layer in Photoshop « Julieanne Kost's Blog

Could you please post a screenshot with the pertinent Panels (Layers, Channels, Options Bar, …) visible?

Adjustment Layers with Layer Masks would be one option.