Research how to offer downloadable textbooks to PDAs or even how to do podcasts of textbooks. Sort of like an audiobook except downloadable to PDA. Textbook rental is an ok idea, however, you`ll be buying the textbook, renting them for maybe 4 semesters, and then the required books change for the class. It will be hard to make money. Beleive it or not, there is a financial gain made by the Universities and the professors in choosing which textbooks to use. Another idea is textbook sharing. Find a way for two people to use the same textbook, cut their cost in half, you profit, everyone is happy.
Thinks ahead of eveeryone else. What will make a student`s life easier, cheaper, what will make them smarter, or better looking.
Just a few thoughts.
You didn`t mention this, but I hope you remembered to factor in one very important expense for this business: author royalties.
Failure to pay royalties based on multiple users could and most likely will result in lawsuits from the Authors Guild, the publishers, and the authors. Unlike libraries, you will be sharing these books for a profit, which can leave you open to copyright infringement lawsuits. Make sure you purchase a multiple user license for each book you acquire. You can do this by contacting the publishers of the books. It would probably be in your best interest to buy the books directly from the publishers, along with the license.
As an author and a publisher, I can tell you we take copyright issues very seriously. In fact, there has been a movement (for quite some time) by some authors and publishers (not me or my company) to charge libraries for multiple-use licenses. Most authors do not make enough money to quit their day jobs, so royalties are a touchy subject with them. Good luck with your venture.