I have a flash drive with all of my public domain books on a key ring that I have used to give friends and relatives books. I have also mailed SD card with public domain books to my sister and calibre download link. She was able to install calibre on her computer and load books from SD card to her computer. My dentist also installed calibre to her computer and books from SD card to her ereader.
I wouldn't give anyone more than 2000 books without calibre to manage those books.
https://www.calibre-ebook.com