Lean Change Management by Jason Little changed my life, and that's no lie. I have so many page flags, bookmarks, and highlighted sections that it's nearly unreadable to anyone but me.
Lean Enterprise is also fantastic; it's heavy with a ton of info but I refer to it frequently (h/t @chrismurman)
Phoenix Project...I wish every "manager" and "leader" involved in my current transformation was forced to read this book. I read it in one day and loved the hell out of it, explains the enlightenment and transformative aspects of an agile adoption better than any powerpoint deck or conference talk.
I could go on for hours, I read at warp speed. Vacation last week, finished three books (and no @ryan they weren't trashy love novels)