Any file transfer option that simply copies the file directly will suffice, or you can use a third-party ebook manager such as Calibre.
To transfer the files directly, you can use AirDrop or file syncing options such as Dropbox and Google Drive. Of these, I would personally recommend using AirDrop, as since it is transferred with a local ad-hoc Wi-Fi connection, it never needs to leave your local network and allows for fast transfer speeds of even very large files. Note that this will only work if your iOS and Mac devices meet these requirements.
The Apple support website offers these instructions for using AirDrop to send files to an iOS device:
- Choose Go > AirDrop from the menu bar in the Finder. Or select
AirDrop in the sidebar of a Finder window.
- The AirDrop window shows
nearby AirDrop users. Drag one or more items to the recipient's
image in the window, then click Send.
Or use the Share feature:
- Click the Share button, if available in your app. Or Control-click an item in the Finder, then choose Share from the shortcut menu.
- The Share menu lists several sharing options. Choose AirDrop.
- Select a recipient from the AirDrop sheet, then click Done.
If you'd rather have a library of all your books on your computer, and then sync them semi-automatically, this answer gives some information as to how to use Calibre with an iOS device.