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.
I don't have a ton of storage. My Raspberry pi400 which is my desktop has a 1TB and a 500GB SSD attached. I keep the Books folder synced with Google Drive along with my phone and Kindle Fire. My Book dir is: 58GB and has 18748 items (16970 files, 1777 folders). Gotta love zlib and Anna's.