The phrase “Leenheer met boekenwijsheid” translates to “Bibliothecaris” in English, which means librarian.
This answer is confirmed by a Dutch online dictionary and cryptogram solver Mijnwoordenboek
The Dutch phrase “Leenheer met boekenwijsheid,” which literally translates to “lord with book wisdom,” is a clever wordplay for “bibliothecaris,” the Dutch word for librarian. This metaphorical description highlights the librarian’s role as a guardian and provider of knowledge, similar to how a lord might have ruled and overseen a domain in the past.