Try something like this. In order to achieve what you want, Library has to be the owner of the association
We should put some custom flow to achieve your use case.
In the Library New/Edit Page Add a button Say it AddBooks and then call a microflow (Act_Library_SelectBook)which will a call a new page with a dataview in iit and Library context object.say it as Book_Select.
Now in the Act_Library_SelectBook place a show page activity and call Book_Select page. Inside the Dataview add a Data Grid with Xpath and context object as Book. In the data Grid Add a Action Button and call a microflow and name it as Act_AddBook_Library. Now the Act_AddBook_Library will have two paramter Library and Book. Put a Change object actvity and map Book Entity. Now in the Association add the Library object and select refresh in client check box and then put a close Page Activity. This will work as you mentioned.
In order to verify in the Library New/Edit page add a DataDrid below the Add button and select Asociation as datsource and Show Book Details.
Add a closepage activity after the Change object.
Let me know if it work for you ATB !!