Membership system with Pressbooks
It requires to integrate PB with RCP (https://github.com/restrictcontentpro/restrict-content-pro/releases), a much simple option than woocommerce.
As PB is a multisite, the activation of RCP would be for the full network, so it will share the settings to all the books: https://docs.restrictcontentpro.com/article/1584-using-restrict-content-pro-with-wordpress-multisite.
- Also is possible to make the payment in one place for the full installation.
A new theme is necessary, so we can protect content or we can protect the downloads just for members (https://docs.restrictcontentpro.com/article/1595-restricting-content-in-template-files)
The first problem we have is: all the administrators can see the plugin settings. So it requires a way to block to the administrators the RCP settings.
The second problem we have is: just administrators can receive the payment, so a distribution of the payment system is necessary (should work multisite too) in order to pay commissions to the publishers (a place for the bank account information for the publishers is necessary in the profile or book info). (examples; https://easydigitaldownloads.com/downloads/commissions/ - https://wordpress.org/plugins/edd-commission-fees/)