Originally Posted by Ortep
In this way she always has the latest version of the library and cannot by accident delete or change books.
If she reads this, you are dead!
Back on topic.
The Dropbox site says:
All members of a shared folder have full read and write privileges. Setting read-only permissions isn't currently possible, but the feature is on Dropbox's roadmap.
Seems a very, very strange omission from a file sharing site.