Hello there, LowTexan.
I was able to locate a good article that gives a great overview of the pre-order process for iBooks.
Particularly the following:
When a pre-order item becomes available, you will receive an email letting you know your pre-order is available for download.
If you've enabled iTunes to automatically download prepurchased content, then your pre-order content may already be downloaded to your library. If not, simply click the download link provided within your email notification, and your pre-order content will begin to download. Alternatively you can use the "Check for Available Downloads" feature in iTunes on your computer by choosing Store > Check for Available Downloadsto begin downloading your pre-order.
Thanks for coming to Apple Support Communities to find answers to your question.