Gutendex vs Open Library

Gutendex is a free, open-source JSON API — built independently of Project Gutenberg — that wraps its roughly 75,000 public-domain ebooks with searchable, paginated metadata: authors, subjects, bookshelves, languages, and direct links to every available download format from plain text to EPUB and Kindle. No account or key is needed to query it, and the project's own source enables cross-origin requests from any domain rather than restricting them. Anyone needing sustained high-volume traffic is asked to run their own instance of the open-source server instead of hammering the public one.

Open Library, an Internet Archive project, exposes its book catalog for search, work/edition/author lookups by identifier, subjects, and public reading lists, returned as JSON, YAML, or RDF/XML, with a separate Covers API serving cover art by ISBN or Open Library ID. No API key is required, though sending a descriptive User-Agent and contact email triples the default 1-request-per-second anonymous rate limit. Bulk access is deliberately kept off the live API — the project publishes free monthly data dumps for that instead.