The phrase "book files" refers to electronic documents or digital versions of books that can be read on a computer, tablet, or e-reader instead of a traditional printed book. Full definition
You can decide to do your own editing, formatting and page layout and send to your publisher your entire book files in PDF format. (authors.com)
Believe it or not, we have receives perhaps 10,000 messages, over 37 years, encouraging us to check certain parts of book files for errors. (gutenbergnews.org)
This issue also highlights the problem with selling one book file for use on every eBook device. (kindleformatting.com)