Habits
Atomic Habits
The clearest manual I've found on building habits that actually stick — tiny changes, compounding results.
Why it's on the shelf
This one changed what I do, not just how I think. The core idea — get 1% better and let your systems, not your goals, carry you — is simple, but Clear backs it with mechanics you can apply the same day.
The 'make it obvious, attractive, easy, satisfying' loop is the part I keep reusing, for everything from the gym to side projects. It's the rare self-help book that survives contact with real life.