On earlier iOS versions of iBooks (now Books), there was an icon one could click, and if the book had an update, the “click” updated the book.

I presume the new Books likewise has this feature, but I can’t find it.

Is this a case of that feature no longer being included, or is Operator Error at play again?

I hadn't thought of it before in relation to Books, but my books in Books appear to be updating on their own without any action on my part. That would fit with the way iTunes/Music handles tunes purchased from the iTunes store and the App Store work in Mojave and iOS 12. The App Store and OS auto downloads and installs can be selected in System Preferences > Software Update > Advanced in Mojave and Settings > General > Software Update > Automatic Updates in iOS 12 but AFIK there automatic updates are not optional in iTunes/Music or Books.

"I hadn't thought of it before in relation to Books, but my books in Books appear to be updating on their own without any action on my part."

I don't doubt/question any of that Joe, It's just that I wonder how one was supposed to have known that. Can it be because it is the modus operandi?