TL;DR

For more info: https://www.bookstackapp.com/docs/user/content-overview/

Embedded images are public and accessible by anyone (e.g. by direct link) 
Attachments are private, access is defined by parent page permissions.
If a user has the right role, and that role can access the page, then they can view/download attachments.
Shelf permissions do not cascade to books, since a book can be in multiple shelves.

Deleted pages are placed in a Recycle Bin and can be restored after a query to @Admin

Revisions are fully visible by anyone with access to page (e.g. public users)
If a revision was made to occlude information, please remember to clear the revision history. Eventually

Pages published are visible immediately - Hold the page in Draft form until you are ready to display it.
There is no similar option for Books or Chapters. Eventually