Step 1 - Measure the space you need to fill.
Step 2 - Pick your colors and style. If you need help run some Google Image searches. You can always find books to decorate with at your local Goodwill or Thrift store.
If you need larger amounts of certain colors or styles you can also buy from The Book Bundler. They are comparably priced to most goodwill's and have a far greater selection than you can typically find at one store and they even offer FREE Shipping. It's saved me a lot of hassle just buying from them direct. I've not found one other service that sells even close to their prices. They also offer custom bundles if you need that extra special look. Be sure to check them out if you haven't!
Modern Style - Books published in 1975 or after. The ISBN was in general widespread use by 1970, and by 1975 the major publishers had all adopted a level of standardization. Modern books are typically either 8 inches or 9.5 inches tall, and will have modern fonts as well as bright paper coloring on the sides
Pictured Above - Modern Black - Only two sizes are typically published on modern hardcovers
Vintage Style - Books published around 1975 or before. With vintage books you see a much greater variation in heights, fonts, coloring, and typically a level of wear that just can't be faked on modern books.
Pictured Above - Vintage Shabby - Many different heights, shapes and sizes
Step 3 - Arrange!
(Source - Studio Mothers)
(Source - Slate)
(Source - The Book Bundler) seriously check them out!