books
Wikisource
Wikimedia's library of free-content source texts — transcribed books, documents, speeches, and historical material — running to millions of pages. You can search it through the standard MediaWiki API or on the website. It is useful for finding transcribed, citable full texts of public-domain and primary-source documents.
Books API
Why it’s useful & how it works
Standard MediaWiki API, no key (set a descriptive UA per Wikimedia policy). Works both ways, fast.
What’s inside
Millions of text pages.
API access
https://en.wikisource.org/w/api.php?action=query&list=search&srsearch=...&format=json
Access
Programmatic API access (a key may be required — see the API tag).