Import https://open.umn.edu/opentextbooks Open Text books

I found a source of freely-available textbooks that are not indexed in OpenLibrary. This issue is to consider including their corpus in Open Library. Actually a short web search brings up several sites that announce freely available texts.

### Tasks

There are numerous sources, I'm not sure which it was that I was checking out:

See https://open.umn.edu/opentextbooks, https://oercommons.org/hubs/open-textbooks, https://collegeopentextbooks.org/, https://openstax.org/,

RefersTo: TrustedBookProvider

Compare the listed sources—Open Textbooks, OER Commons, College Open Textbooks, and OpenStax—and determine which corpus should be considered for Open Library. Start with the TrustedBookProvider reference and existing project guidance for source imports. Done means selecting a source, defining the corpus and import scope, and documenting the required approach.

Written by the indexing model from the issue text.