Just remember, if you buy Star Trek eBooks from Apple, you've locked yourself in big time. You cannot strip the DRM. You cannot view them without iBooks.
But, if you use the txtr, Bluefire, or B&N apps, you can buy from other shops and strip the DRM. So your investments are not lost if you ever move away from Apple.
The ideal option for me would be if I could just buy the e-books from somewhere, strip the DRM and convert them to txt files. Then I could just read them using my existing screen reader software.
Unfortunately, for me, I have no idea how to go about removing DRM.