iBooks does not seem to be recognizing that books have been previously purchased when the purchase price has changed.

You are totally wrong. I spent a good deal of time on the phone with Apple Support's iTunes group on this yesterday.... it is not what you surmised... Somehow the book - and 50 others! - got " hidden" during my Mojave upgrade.

Apple for some reason had to separate Books from iTunes so if you want to find your hidden books, you now have to do two things... first, Books -> File -> Move Books From iTunes. I believe you only have to do this one time.

If they are still not showing, you have to go into your Apple ID account from within the Books app. (not within the iTunes app!) Book Store -> Featured ->Account and scroll down until you see "Hidden purchases" and then click on "Manage" and you will see your hidden Books and be able to unhide them. [For whatever reason, if you attempt to do this from within iTunes' account page, it will not show your books, it will only show your hidden iTunes stuff..]

Truly I don't know what Apple was thinking when they redesigned Books... what used to be intuitively and easily accessible from a single screen (select, remove, moves, etc.) now takes backing in and out of multiple screens and you still can't for example find a list of just your books that aren't on your device. It is very difficult, if not impossible, to select multiple books to perform a single action now. Turns out that 51 of my books were "hidden", not just the one I was particularly interested I, and my only choice was to unhide all, or go through them and unhide one at a time.. If you are trying to remove a book from your device now, you can only remove one at a time, not a selected group anymore) and to make matters worse after you remove each one the screen repaints to the top of the page...

That is an absurd answer.... the book had originally downloaded and then it disappeared... I didn't RENT the book, I bought it... what right do they have to take it back unless they want to refund my money????

Thats fraud.... Are you telling me that this is officially Apple's policy???

But I didn't hide them... according to the support person I spoke with, its been happening because she gets a lot of calls bout it and she thought it was because of the move of the book purchases from iTunes to Books which for some reason doesn't always complete in the upgrade. In any case, what you are in error about is the fact that "they will be treated as new purchases"... that's a bug, not a feature...