Plus, shelves have two (possibly three) dimensions, so you can have organization running vertically and horizontally.  The third dimension is into the bookshelf, if you have a bunch of double-shelved paperbacks.  I have <i>Jurassic Park</i> shelved next to <i>Frankenstein</i>, with Tolkein up and to the right.  <i>The Lord of the Rings</i> is in front of <i>The Silmarillion</i>, <i>The Hobbit</i>, and <i>Unfinished Tales</i>.