"Professional" probably in terms of "professional development", i.e. the ideas about character and historical knowledge deemed necessary by the military. I thin

"Professional" probably in terms of "professional development", i.e. the ideas about character and historical knowledge deemed necessary by the military. I think that each branch has its own reading list, and the book lists are "picked" and revised by the most senior leadership (and also most likely their staff). For example, the Commandant of the Marine Corps's reading list (https://www.usmcofficer.com/marine-corps-knowledge/commandants-reading-list/ https://www.usmcofficer.com/marine-corps-knowledge/commandan...), which includes books for each rank for both enlisted and officers.