An evergreen oldie (2,400 years give or take) that emerging leaders have read for centuries is Xenophon's Anabasis often also titled The Persian Expedition. The work is an account of an ill-fated expedition by a small Greek army, the legendary Ten Thousand, to Persia to assist Cyrus claim the Persian throne. Cyrus is killed, the Greeks loose all their senior commanders, and they have no choice but to march all the way back home, through VERY hostile territory, with the Persian armies always at their heels!! The book features many very memorable lessons in leadership and styles of leadership. A cracking, accessable read, that is still very very relevant, as it is essentially about the human spirit. I have the Rex Warner translation published by Penguin and well worn it is! There are editions online but they feature older, less accessable translations.