Project Gutenberg
Project Gutenberg began in 1971 and is one of the oldest digital library projects in existence. Volunteers digitise, proofread, and archive books whose copyright has expired, giving you full-text works you can use without licensing worries. The collection is mostly English, with roughly 55,000 English-language titles, alongside thousands of books in other languages.
You can download individual books as plain text, EPUB, HTML, or Kindle files, or pull the whole catalogue in bulk. The plain-text editions are the easiest to chunk (split into passages) and embed for retrieval, though be aware that older files include a standard header and footer you will want to strip out first.
Copyright status is handled per work rather than across the whole collection, so a title that is public domain in the United States may still be under copyright elsewhere. Check the status of each work for your jurisdiction before you build on it.
Related sources
Google Books Ngrams
Word and phrase frequency data drawn from Google's digitised book collection, spanning centuries of published text. It counts how often words and short phrases appear by year, which is useful for historical language analysis and time-aware RAG features.
LibriVox
A volunteer project providing free public domain audiobooks. Volunteers record themselves reading works whose copyright has expired, which makes it a handy source of speech data for training speech-to-text models and building multimodal RAG.
Open Library
An Internet Archive project building a web page for every book ever published. It holds more than 20 million catalogue records and lends many titles digitally, making it a rich source of book metadata for RAG systems.
Standardized Project Gutenberg Corpus (SPGC)
A research-ready version of the Project Gutenberg catalogue with consistent formatting, tidy metadata, and token counts for every book. It saves you the cleanup work, so you get uniform full-text literature ready to chunk and embed for RAG.