Book files are tiny though. Even very long books with a bunch of pictures are only usually a few megabytes in epub format. A $5 USB stick should fit thousands of books on it.
If you’re using PDF for books, then stop doing that :) PDFs have a bunch of limitations - the main ones being thay you can’t change the font size, and it can’t reflow the layout based on your screen size (i.e. format the book so a page exactly matches your screen size). They’re hard to read on mobile as a result.