The US Army has an exceptionally well-crafted field manual on leadership. No fluff, but also no chest-thumping. It's honestly the best resource I've ever seen on what constitutes a good leader, how good leaders are developed, and how good leaders develop their teams.
Being a DOD publication, it's in the public domain: https://usacac.army.mil/sites/default/files/misc/doctrine/CD...
I can't stress enough how excellent it is.