The answer to your question really depends on what you need it for. Technical or content writing? Work communication? Deeper conversations in your personal life?
I think the most broadly applicable answer is to read the most famous English stylists such as Gibbon, Carlyle, Arnold, or novelists like Austen, Eliot, Henry James, etc. This writing is complex and sophisticated enough to be challenging even to native speakers, but the vocabulary is still almost totally congruent with modern English.
However, since you're posting on HN, I suspect you'd like a response from a more technical perspective, in which case my answer is: read and write open source documentation! Writing good docs requires complete understanding of the terminology and concepts involved, as well as compositional skill and precision. It's a great exercise which will improve your English, as well as your understanding of the code you're writing about - and as a bonus you're giving something back while you learn.
my answer is: read and write open source documentation
I disagree. You need very little language skill to read technical docs if you're familiar with the topic, and only slightly more to write them yourself.
I think the most broadly applicable answer is to read the most famous English stylists such as Gibbon, Carlyle, Arnold, or novelists like Austen, Eliot, Henry James, etc. This writing is complex and sophisticated enough to be challenging even to native speakers, but the vocabulary is still almost totally congruent with modern English.
However, since you're posting on HN, I suspect you'd like a response from a more technical perspective, in which case my answer is: read and write open source documentation! Writing good docs requires complete understanding of the terminology and concepts involved, as well as compositional skill and precision. It's a great exercise which will improve your English, as well as your understanding of the code you're writing about - and as a bonus you're giving something back while you learn.