Unfortunately we do not have wall to wall bookshelves.
I use this idea to store books in the girls room.
image from Pottery Barn
They do have a bookshelf in their room, but too many books. Using storage baskets (these are from Pottery Barn, mine are not this cute) on the bookshelf allows you to store more books in the space provided. It also works really well for books that are too big to stand up on the shelf.
Here is an idea that a neighbor of mine uses. (Although this isn't her house...she has this in her girls room)
image from lagomorph
You can use rain gutters to make an inexpensive book display. This works really well if you have wall space that isn't deep enough for a book shelf (like where a door would open). Not only do the books get a home, but you can see all of their cute little covers!
What ideas do you use in your home for books?