In a den or library with bookshelves, how and where you put pictures depends on the height and design of the shelves. If your bookshelves are from floor to ceiling and wall to wall, add framed photos or paintings that are in proportion to the interior spaces provided. If your shelves go only partway up the wall, use the space on top, either hanging or placing pictures on the surface, as well as inside the shelves.
A new trend in decorating with floor-to-ceiling shelves is to use the shelves themselves to hang wall art. For the best effect, hang a very dramatic painting or print from the center portion of shelves, leaving the shelf space behind it empty. While this is not for everyone, it is gaining popularity.
The best part about wall-mounted shelves is all the opportunities they offer you to create interesting wall treatments. Whatever your style -- traditional, contemporary or modern -- you will find shelves to complement it. Mix up the lengths and widths of the shelves for added interest. Some come with grooves so you can create a lovely wall by framing black-and-white family photos in black studio frames and displaying them on the shelves.
A very dramatic wall can be made by framing your sofa with shelves on each side and above it. For example, if your theme is nautical, hang a large painting or print of a ship or the ocean over the sofa. Place smaller marine-themed pictures among your books on the shelves.