This is a pretty good list of recommended reading published by the Infantry School. It's broken up by rank, and is a good indicator of what the Army considers important concepts for development of its soldiers. I would think any of the books for an Lt. would be appropriate.
War and Peace was one of my favorite books to read all time. It took 6 months for me to get through it, but it was one heck of a read(assuming they haven't read it yet).