I just tried dragging a purchased book to Finder, and it only gives me a .webloc pointing to the book's store URL. On the other hand, if I manually locate the book in ~/Library/Containers/com.apple.BKAgentService/Data/Documents/iBooks/Books, it appears to just be an (unzipped) ePub without any encryption.

I just tried dragging a book out of the “library” tab to my desktop and it gave me an epub. Tried on macOS Ventura with a book released this year

It's up to the publisher whether content bought from the Apple store has DRM, not the store itself. Many books have it, but publishers like TOR and any books from Brandon Sanderson don't. There's sometimes a line at the bottom of the description that'll note it. For example, on Redshirts (https://books.apple.com/us/book/redshirts/id501758516)

> At the Publisher's request, this title is being sold without Digital Rights Management Software (DRM) applied.

Go to an Apple Store and do it, or buy an old MacBook Air / Mac mini for ~nothing off eBay.