i don't know if they have them at gamestop.
but i bought ghosts of onxy at walmart, and first strike i got from the library. (first strike tells of everything that happens after halo 1, and how halo 2 comes about) and the other books is just filling you in on the story.
ghosts of onxy (i think i'm spelling it wrong) is what's happening between halo 2 and halo 3. (which is why i think X will be the jackel shield in some levels, hitting the X button will keep the shield charged / charging while your being fired upon. if that makes sense) anyhow, i would suggest getting them, they aren't that long of a read, and i got down with both books in little less then 2-3 days for each book.