i never have a book before, but the question is does the book disappear when you read it?
probably they deny the cancelation is... probably to simplify and ease on counting the use of the book
Just a technical thought, and this is only my guess about how it works behind the scenes.
I'm assuming a book is tracked with two things...
X = how many copies you own
Y = the reading timer (start or end time, like 5 jan 2026, 15:00:12 TCT)
Right now, pressing Read probably does: start Y, and X − 1. When Y finishes, the effect applies.
Cancel adds a new case: pressing it clears Y (and maybe X + 1 if the book is returned). That's simple on its own, but it opens edge cases. On a bad connection, or if someone double-clicks, the cancel could be processed twice and give X + 1 twice, or Cancel and Read could collide.
To prevent that, the devs would probably need a flag (Z = 1 while reading, 0 when not) and check it on every click. That's doable, but it's one more thing to track and test, which might be why cancelling isn't there.
I'm not a dev though, so this is just my assumption...