Digital Public Library of America (DPLA)
The Digital Public Library of America pulls together records from cultural institutions across the United States into one searchable collection. The API covers two things: items, meaning single physical objects indexed by a data provider, and collections, meaning logical groupings of those items. Partner data comes from organisations including Harvard, the New York Public Library, ARTstor, and the David Rumsey Historical Map collection.
If you want the full dataset rather than API calls, all data in the DPLA repository is available for download as zipped JSON files, which makes bulk processing straightforward. The metadata is released under CC0, so you can reuse it freely, including in commercial work.
For RAG, DPLA is a clean, well-structured source of descriptive metadata about American cultural objects, useful when you want provenance, dates, and subject information sitting alongside your other text.
Related sources
Common European Data Space for Cultural Heritage
The umbrella project coordinating cultural heritage data across Europe. Led by the Europeana Initiative, it helps cultural institutions and EU Member States share their collections through a common, shared infrastructure.
Europeana
A platform for discovering cultural heritage collections across Europe, with multilingual access to over 60 million digitised items from around 4,000 institutions, including books, paintings, maps, manuscripts, and audiovisual and 3D media.
HathiTrust Digital Library
A partnership of academic and research institutions offering millions of digitised titles. Its Research Center provides computational access to the full corpus, including in-copyright works, under non-consumptive research terms.
Smithsonian Open Access
Millions of digital items from the Smithsonian's museums, research centres, and archives, released under CC0. Includes images, 3D models, and research datasets you can reuse freely, including commercially.