Click on the little relationship box - there are options about "Allow creation of records in this table via this relationship" and a deletion option.
Also the Author field in Books Table is an ID or text field unique to the book.
Most often developers will have unique IDs created when record is created.
They often name the main table ID "MainID" and the other table "OtherMainID" for clarity.
The first five links in this list are very helpful when starting FMP design