Hello and a warm welcome to the iPadForums
I'm not personally aware of any actual text books that are currently available on the iBookStore just yet. I have seen much news as well as rumors surrounding the text book industries and hope that it is something that comes to fruition soon, if not for the folks that have to purchase and carry individual books about, but for those who also foot the bill for those books.
The convenience alone will be a great benefit and along with the multitasking support arriving soon with iPhone OS 4.0, working from a book while performing other duties such as note taking etc, will be slicker than ever on the iPad.
I am in both IT and Service Repair businesses and perform all kinds of daily duties as well as routines which one day may require one way of doing things as the next day requires another. Performing tasks on the iPhone was a great freedom from desktops as well as laptop use for me while out performing my daily service contracts. I was even able to create now on the iBookStore all my Laser Printer Manuals and have them at my disposal as e-books. So I am still migrating many of the apps I used so arduously on the iPhone to now use on the iPad and as the new versions arrive I am happily updating them to the better looking iPad device for me and my customers.