How to restrict what content types can be added to a Book?
The answer seems to be within the permissions. For books there are a few main permissions (plus others in the Node section):


Administer book outlines
Create new books
Add content and child pages to books


Removing the 'Administer book outlines' will allow roles who have the other two permissions to create book pages but not see the option to add other content type nodes into a book. This is exactly what I was looking for.