Hathitrust is another wonderful use case of Solr. It has almost a couple of terabytes of index, with more than 10 million books provided online. Solr plays a prominent role in searches through its huge library of books. There are many such examples having similar use cases:
- Internet Archive: https://archive.org/
- National Library of Australia: http://trove.nla.gov.au/
- FictFact: https://www.fictfact.com/
- Biblio: https://www.biblio.com/
- A Norwegian online book store: https://www.akademika.no/