Certain things just go together, and in my mind, mantels and bookshelves are akin to bread and butter and PB and J.
Even if the shelves aren't totally filled with books, there is something very smart about the combination. It's timeless and practical and suitable for pretty much any personal style, be it more traditional (Image 7), a little girly (Image 8), or cleanly modern (Image 5). How you organize your tomes is up to you (ahem, color-coders), but with your books flanking the mantle, the final effect is bound to be good.
Images: 1. Annie Lou Berman for Apartment Therapy 2. Design*Sponge 3. Jerry Jacobs via Despire to Inspire 4. Design*Sponge 5. Ab Chao 6. Miles Redd via Matters of Style 7. Simon Upton for Elle Decor 8. Country Living via Matters of Style