Francis Bacon, one of the greatest thinkers in history, wrote these insightful words, "Old wood is best to burn, old wine best to drink, old friends best to trust and old authors best to read."
How can you claim to be a competent manager and not be continuously reading something by Peter Drucker?
Drucker is gone now but his influence grows. He was the father of modern management and the first genuine leadership mentor in the western world.
Google him. Click on anything he has written. Read it. You will be a better manager in days and a better person immediately.