Greg McKeown

Essentialism argues that the useful skill is not doing more efficiently but deciding what not to do at all, and that saying yes by default is how capable people end up busy and ineffective.

The line that carries it is that if it is not a clear yes, it is a clear no. Effortless followed, on making the important things easier rather than grinding harder at them. McKeown was a consultant and the examples come from that world, so the advice assumes a degree of control over your own diary that not every reader has.