Don’t despair, there are workarounds for this. See the many answers to https://apple.stackexchange.com/q/137047/21473 ‘Don't want iBooks to always paste the “Excerpt From” of what I have copied’.
I recommend the solution in https://apple.stackexchange.com/a/382603/21473 (which is an improved variant of stassats’s solution in https://news.ycombinator.com/item?id=22355322). The method is to use Automator to set up a Quick Action that copies the text while bypassing the clipboard-modifying behavior of Books. Then you configure a keyboard shortcut of ⌘C for the Quick Action, only in the Books app, so you don’t have to change the way you copy.