Graphic Design the New Basics, Lupton
Layers used to be made to make maps and come from and are used in most printing techniques. Digitally it gives us the option to used all types of images (referring to the image types in my last reading respond post). Layers are useful for displaying all sorts of ideas, feelings, or stories. Even without images, type can make an impact layered. In my book, type will push the idea of the images (dots) that will be composed on the following page. It can overall impact the aesthetics of my book if all forms are consistent in the matter of structure, direction, etc.
Transparency can be used to show the passing through another shape and also emphasizes different layers. Transparency and layers are related, they share a relationship. All of these principles apply for images and type as well, not only separate but together as well.