There are a number of good books for a new lieutenant, some have already been mentioned. If I could recommend only one, it would be Words for Warriors: A Professional Soldier's Notebook.
https://www.amazon.com/Words-Warrior.../dp/1587368056
COL Puckett is a national treasure when it comes to military leadership and I consider myself fortunate to call him a mentor.
https://en.m.wikipedia.org/wiki/Ralph_Puckett
Watching an 80+ year old man lay down on the cold ground with a machine gun team at 0200 on a cold winter's night to discuss the finer points of employing a machine gun- not because he has to, but because he wants to- is a special lesson in leadership. There are few better men to learn leadership from.