If you have shared a book's URL as an online link and want to remove access to the book there's a couple of ways you can do it.
If you no longer need to keep the book active you can archive it from your account. Click on the 3 dots below the book to open up its menu. From there you can Archive it.
If you want to keep the book but just remove access until you choose to share it again, head to the Books screen and select the three dots next to the book you want to remove access to. Then select 'Copy' followed by 'Copy with Coverage' in order to create an exact copy of the book in your account.
To remove access to the share link of the old book you'll now need to Archive the book and remove it from your account. Select the 3 dots again and this time select 'Archive'.
You'll now have an exact copy of your book but with a new URL.