What a great question, Elizabeth! I used to have two rooms lined with bookshelves that I bought from Target or OfficeMax, but it all got to be a bit much. Especially since the bookcases scuff up the walls, and are not exactly easy to switch around.
Then I reduced my book collection to the essentials, and bought a bookcart to put them on. This is the same kind of bookcart you see in school libraries. I got mine in a great color (royal blue, from alfaxfurniture.com, if you're interested). It's got sturdy wheels so my collection is now portable if I want to dust underneath it, or take it to another room.
The bookcart itself has storage space on both sides: three shelves that are 36 inches long, and slanted at a slight angle so your books don't drop out. I'm going to try to post an image of it here so you can see what I'm talking about:
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