Hi Adrija,
Through your explaination, I understand that you have requirement where librarian needs to approve a book requested and email needs to be triggred. In such case you will need to create a microflow.
broad overview of how it can be achieved:
1. you will need an attribute to hold the book status(it can be enumeration with values like "requested","approved","pending" etc). and User with role "librarian" can modify this attribute.
2. Following to that you can create a flow for email triggering(refer this Email | Mendix Documentation)
Hope this helps!