There are two small management books that have had an enormous influence on me. They didn't get a lot of attention when first published - at least not as much as they deserved - but I find that I keep returning to them to refresh my perspective and stir my thoughts.
Management of the Absurd: Paradoxes in Leadership by Richard Farson, and
Thinking About Management by Theodore Levitt.
These are very deep wells and you'll draw from them often.