Add child pages

I agree

A user can have the ability to edit a book but not add child pages.

In fact, users can add child pages if:

• they have permission to add pages to books, or they have permission to administer book outlines;
• AND
• they have permission to create a node of the content type set as book child content type;
• AND
• they have permission to edit the book currently visualized.

Users with permission to administer nodes always can add pages to a book.

I have the same issue.

In my setup users should generally be able to see and contribute to certain books, but not even see certain other books.
If I setup the permissions to allow a role to add content to a book, automatically I allow that role to also edit content posted by other users.
If I take off the permission, the 'Add child page' link disappears.
I understand the intended meaning of granting update access to role/group in the wiki-like spirit of "let's contribute to the book all together".
This is questionable in a different setup where users contribute and a moderator/admin oversees the process, like building a library of selected content.

I am working on a small patch that I will submit once I get around to learn how to do so.

Here it is.

I hope this was done correctly.

I know it would have been better to have the setting at book level, but... I wanted to keep this simple for now, waiting to see how it is accepted.