Passion of Command by Col B P McCoy USMC for quite some time. I'll admit that I read the kindle edition, which is very condensed (I ripped through it in a day), but in it's 88 pages it covers an enormous amount of ground.
I am an abiding admirer of the USMC, and this book was recommended to me by a former member of the Corps. My experience of Marines is that they are in the main highly educated, driven and passionate individuals. The Corps seems to generate intensely thoughtful people, many of whom are extremely widely read and able to deploy that knowledge at will. Col McCoy typifies that type of character, and from that standpoint this book is a delight to read. Well informed, well sourced and elegantly written. Strip away the military language and much of this book is absolutely right for civilian leaders.
The central contention is that the provision of strong leadership is in itself a moral act. The implication (never expressly stated) is that a leader who doesn't seek to become the best leader they can be is acting in an immoral way. That's a powerful statement, and contains a raw challenge to a great many leaders who assume their roles by simple advancement within an organisation, relying on their instinctive skillset to carry them through.
Something this book also captures is something that I think isn't unique to the USMC, although it is one of the better organisations when it comes to inspiring it. That is an ability to generate in it's leaders, at all levels, a broad dissatisfaction with any form of status quo. Again, it is the moral imperative of a leader to seek to improve the likelihood of those they lead to succeed, again, as true in military as civilian life.
Highly recommend this book to anyone interested in the concept of leadership. This book explores it through one angle, that of a military leader, but much of the conceptual side is entirely relevant to a civilian.