Here is what I do... I always buy my e-books in Microsoft LIT format, either from Fictionwise, Amazon, or eBooks.com. I then use Convert Lit to convert this e-book (which is protected by DRM) to HTML (of course only for personal (fair) use). With HTML, you are free to do what you want, e.g., convert it to a format appropriate for the iRex.
See also
here for more on Convert Lit.