How do I add a constraint after I've truncated a table?
Use WITH NOCHECK instead of WITH CHECK to add a constraint that conflicts with existing data. https://stackoverflow.com/questions/529941/whats-the-difference-between-with-check-add-constraint-followed-by-check-constr

Perhaps you are getting the error because you "2. Trucate a table." and not all of them? Maybe you are removing the Authors or the Books and are leaving the records in AuthorBooks that references these other tables.