Google Books API
DegradedNot yet verified
Searches Google's book catalog for metadata, previews and availability by title, author or ISBN.
Example request
curl -s 'https://www.googleapis.com/books/v1/volumes?q=isbn:9780747532699&key=$API_KEY'
Free tier, in plain English
No published quota or pricing page; docs say a request for public data needs an API key (or OAuth token). A key-less test call returned a shared/default-project quota error rather than a hard block.
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