Apple's new iBook Author may be free (as in beer), but it relies on other software, such as Pages and Keynote which are not free.
The principal issue here is that textbook publishers who wish to make rich content eBooks need to think carefully about the tools they use to ensure that their product reaches the widest audience. Going down the Apple route is dangerous. Personally, I think the best approach is to use an application that generates an eBook format that can be read on any platform.
On the other hand, I strongly suspect that a 1300+ page graphics rich textbook like Molecular Biology of the Cell is going to exceed the capacity of an iPad. not to mention the impossibility of replacing the experience of leafing through the book. Don't forget that students generally use textbooks as reference, and read them differently than they would a novel.