This necklace comes with whichever Harry Potter book you wish. The book it comes with will hang loosely on the chain, but if you buy additional books with clasps, you can clasp them wherever you wish on the chain.
There are two chain sizes. The long chain is 27″, long enough to fit over your head without a clasp (most likely, so a clasp is included anyways). The short chain is 18″.
Note: I did not take pictures with each book. You get the idea. You can see what the book charms look like by visiting their various product pages in the shop.