His 1973 tome Management: Tasks, Responsibilities, Practices, The Essential Drucker, and Managing for Results are three I find myself opening regularly.
The Effective Executive is great as well. It's hard to narrow down because he was such a prolific writer. Recommendations are also hard because you've got to meet the reader where they are. I picked up and put down Drucker early in my career. Years later, the same pages burst with insight when I read them.
