HathiTrust Digital Library

HathiTrust is a shared digital library built by a partnership of academic and research institutions, holding millions of digitised books and other titles. Public domain works can be read in full, while access to in-copyright material is more restricted.

The HathiTrust Research Center opens up the whole collection for computational work through non-consumptive research: you can run analysis and text mining across the corpus, including in-copyright works, without being able to read or extract the original text directly. This lets researchers study patterns across millions of books while respecting copyright.

For RAG, this is a research resource rather than a source of freely reusable text. Public domain titles can be used more openly, but anything in copyright is available only under the Research Center's non-consumptive terms, which rule out commercial reuse. Check what applies to the specific titles you need.