Can iBooks be read without a data connection?

Yes, iBooks can function normally while not connected to the internet. As long as you have the book(s) present in the iBooks app before you get out of coverage you will be fine.

If you are out of your coverage area and connect to another network, or roaming, you might have additional expenses for using another network.

1. Transfer the books from Mac to iPad/iPhone via iCloud.
2. Use Files app to access the books from iPad/iPhone. Open them in iBooks and download them from cloud while connected to internet.
3. Once it's done, go to iCloud location in Files app, select all the books and move to the "On my iPad/iPhone" location (for eg., Keynote folder). Turn off internet and see if you can open the books from "On my iPad/iPhone" location.