Comment 2 for bug 1748537

The open library covers page does mention rate limiting resulting in 403 Forbidden responses.
https://openlibrary.org/dev/docs/api/covers

They say they allow 100 requests/IP per 5 minutes. I can see a site hitting that easily with a few people searching/browsing at once.

They also state some guidelines that evergreen doesn't follow.

If you want to display covers on public-facing pages, please use a src URL that points to covers.openlibrary.org. For example, if you'd like to call a cover using an ISBN, you can do it like this:
<img src="http://covers.openlibrary.org/b/isbn/9780385533225-S.jpg" />

A courtesy link back to Open Library is appreciated, whether it be on each individual book's page (where you can link back to the book's page on Open Library, for example, using the same ISBN http://openlibrary.org/isbn/9780385533225), or on your About page or in your footer.