To create a persistent link to a book, go to the book's Detail Page, then click User-added image (to the left below the cover image).

To create a persistent link to a particular chapter or page in a book, use Read Online and navigate to the desired page, then click  User-added image “Share Link” (in the icon bar at the top).

Once you click User-added image "Share Link", the link is displayed and you can copy it:
User-added image

 Note, if you are using a proxied link, the URL generated will also be a proxied link.