Books, book covers and related data.
Penguin Publishing API is a free Books API that requires no API key, is served over HTTPS and supports CORS, so it can be called from the browser.
Details from penguinrandomhouse.biz.

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Web-API for fetching data from Project Gutenberg Books Library
The Penguin Publishing API is provided by penguinrandomhouse.biz (penguinrandomhouse.biz).
Yes. The Penguin Publishing API is listed in the Books category of the public-apis directory, which only includes APIs that are free or have a free tier. Check its documentation for rate limits and paid plans.
No. The Penguin Publishing API requires no authentication, so you can start making requests without signing up.
Yes. The Penguin Publishing API supports CORS, so front-end JavaScript can call it directly.
Yes. The Penguin Publishing API is served over HTTPS, so it's safe to use in production.