The best way to install pressbooks is a fresh multisite. But maybe, there is a chance to bring together some multisites in one installation?
Just recently I found out, that you can build even networks of multisites with this plugin:
Here is a description of what happens then:
I would love to share the user-data-base with my existing blogging multi-site and still have PressBooks in a separate multisite, but in the same networks of multisites. I hope you understand, what I mean.
Did some of you every try, if this could work? It would make it so much easier to create a multi-usage-plattform, where pressbooks is one of the “apps”.