depending on the book license, (if it has editing rights such as plr) I take those files to my convert service, (you can find several online with free use) and convert the files to something I can use or edit. For instance, some of the books are old, and only come with a PDF file (I am assuming this is your issue) so I convert them to a PPTX, txt, or a doc file, and if for some reason that does not work, I convert to jpg, and white out or rearrange what I need in Photshop or another editor, then add those pages into a pptx and create a new product or pdf.
I know this can be frustrating, but sometimes you have to take the long way round. :)
Plan Your Life In Color!