Although it isn’t a book-related website, one can find a vast number of works in the fields of book cover design, layout design, illustration, and visual design for publications.
Design Works Exhibition Community
Click the "Visit Site" button on this page to open the official Dribbble website, then follow its on-screen instructions.
Dribbble is accessed online through your browser, so there is usually no local installation. Refer to its official site for details.
You can browse comparable reading sites under the e-books category on this site and pick the one that fits you.