JSTOR and their publishing partners has made a (growing) collection of more than 35,000 ebooks freely available for Max Planck through June 30, 2020.
https://www.jstor.org/subjects
See the participating publishers at: https://about.jstor.org/covid19/expanded-access-to-ebooks/
JSTOR Labs have introduced “Text Analyzer” (beta), a new way to search for articles and books on JSTOR.
Upload a document, the tool analyzes the text, identifies the topics and generates a set of terms. You can add and remove terms and increase their relative importance. In the end you get a list of recommended articles from JSTOR.