i need a button or a menu link to add a new book
In BookLibrary you can create link to add new book need little rebuild component. But with that you lost backend option for set - for what categories you permit add new books
For customer version you can send request through this site contact form.
my customers upload their own books so i don't need the ISBN nr.
can i configure it that it's not needed to fill in when they file a new book
That need rebuild component.
For now you can set at bookLibrary backend setting "Check duplicate ISBN:" - to NO - with that users will can set it any value.
and "Default host:" -> "On your own" - for input self data