So I'm sure a lot of you are wondering why pre order are on the warhammer I books on iTunes (myself included) but the helpful GW digital guys have said what is about.
via Games Workshop: Digital Editions
You don't have to pre-order if you don't want to, but if you do it on the iBookstore with the interactive edition, it does mean that your codex downloads automatically, as soon as the book is available, which is kind of cool.
Also, whenever we don't do pre-orders, people always ask when they can pre-order it - some people just like to pre-order stuff.
It's kind of like christmas when your present's under the tree: you can see it, and you have a week to get excited about it, making the opening all the sweeter