Some Feedback

I like the basic idea of this module.

Though I would prefer another approach. Some ideas:

• Make any subsite an entity with an entity reference field from the desired node bundle. This would enable a more flexible approch.
• With an entity reference field from the home subsite node it becomes fiedable and fields can be shown on any subsite page or as a block. Permissions per bundle OOTB.
• Have a subsite condition for blocks etc. also great for using context module
• Provide options for each subsite entity bundle which overrides are provided or not. Alternatively they could be removed by the bundle form mode.
• Enable an option to show the subsite page as home and all menu children as child menu items. No books needed here in many use cases. Easy and simple
• Make book and social media modules optional submodules. I donnot like to be forced to use them if not necessary.
• Enable an option to choose a body class via a predifined select list plus the subsite name. Much theming can be done this way, also in combination css vars: :root { --branding-color: var(--red) } body.mysubsite.colorvariant-2 { --branding-color: var(--blue) }

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I currently have a project where this approach would be perfect and save a lot of work. Not sure if these ideas fit into your development scheme.