Downloading and syncing books to your iPhone or iPad via iTunes (or Finder on newer macOS versions) is a straightforward process that allows you to manage your library from a computer. While Apple has largely shifted local media management to the and Finder apps on Mac, Windows users still utilize iTunes for this purpose. How to Download and Sync Books via iTunes (Windows)
To avoid cables entirely, you can simply upload your books to on your computer. Once uploaded, open the Files app on your iPhone, tap the book file, and it will automatically open and save into your Apple Books library.
Once the process finishes, open the app on your mobile device to find your titles. Modern Alternative: Using Finder (macOS Catalina and later)
Choose to sync or only Selected books that you just added. Finalize the Transfer : Click the Apply or Sync button in the bottom-right corner.