They scanned a bunch of books, and then allowed search results to hit them. They then only show a small context around the hit to avoid copyright infringement.
Bibliography:
- www.theatlantic.com/technology/archive/2017/04/the-tragedy-of-google-books/523320/ Torching the modern-day Library of Alexandria (2015) by James Somers
- The Google Story Chapter 21. A Virtual Library paints a good picture of the people involved
www.aip.org/history-programs/niels-bohr-library/ex-libris-universum/harvard-project-physics-role-history-science mentions that they have a special focus to the history of physics, as can be seen e.g. on The World Of Enrico Fermi by Harvard Project Physics (1970).
There are unlisted articles, also show them or only show them.