The Pragmatic Programmer #5 : Invest in your Intelligence or expand your horizons.

This post focuses on one of the core ideology that is around from a long time but is forgotten because of another thing which in surface is counterintuitive to this, Keep learning, in this chapter author gave an anecdote about how good developers know more than what their job entails and keeps learning to expand his/her horizon, I have heard counter argument against this which goes along the line of something like, if you lose focus on learning new things then your core strength will dwindle, it's very easy to miss the point set by author here, he explains that you should expand your horizon and differentiate (another point which was enforced by Deep Work book) in your own field, so losing focus argument fails since author wants to invest in your intelligence, he is not suggesting to travel the world (although that might help in broadening your perspective about life in general) but simply asks to keep learning things which might be beneficial in future but won't really be applicable in your current job.

tldr; learn something which goes against your core competence but not too much, eg., learn a functional programming language if you know an object oriented one or vice versa.