Arrange your books and photos as though the shelves were only 12" deep, placing things at the front of the shelves. But take advantage of the full depth for large items.

Do as CA-KD suggested and for the technique use 1/3 books (standing, stacked or propping other items,) 1/3 large decor items (or smaller ones grouped in threes,) and 1/3 open space. Keep framed pictures to a minimum and layer 2-3 sizes, one in front of the other for more interest than just flattening them out facing the "audience." This way they'll want to approch the shelves to see the "hidden" detail of the pix. You can paint the backs of the shelves in a color to bring them forward visually, and you can insert false backs to actually narrow them if you want.

Thanks, great suggestions.