This term has been covered by the website Wikipedia as "WorldCat"
This mediawiki site, a `wiki dictionary` about the nature and practice of free improvisation declines to duplicate the content of that listing. Below there is an introduction - a simple copy and paste. from Wikipedia then a link to the Wikipedia entry.
Introduction, copied from Wikipedia
WorldCat is a union catalog that itemizes the collections of 17,900 libraries in 123 countries and territories that participate in the Online Computer Library Center (OCLC) global cooperative. It is operated by OCLC, Inc. The subscribing member libraries collectively maintain WorldCat's database, the world's largest bibliographic database. OCLC makes WorldCat itself available free to libraries, but the catalog is the foundation for other subscription OCLC services (such as resource sharing and collection management). WorldCat is used by the general public and by librarians for cataloging and research.
Reason for including WorldCat in this improv wiki dictionary
You can search this database for books on free improvisation. Some title maybe out of print, others published by academic publishers are very expensive.
Link to the Wikipedia page