I believe one of the best ways to improve your vocabulary beyond reading books is to watch movies. Not just any movies, but movies with depth, movies that care about language, movies that have something meaningful to say.
I cannot count the number of rich words and phrases I picked up while watching Black Bag. It felt as though every scene required me to pause, look up a word or phrase, then continue watching. I am currently watching Slow Horses and I have found myself exposed to an entirely different set of words, particularly British expressions and slang.
The trick, however, is that hearing a word once is rarely enough to incorporate it into your vocabulary. You have to actually use it. For example, I recently came across the phrase “hair of the dog,” which refers to drinking alcohol to cure a hangover. A friend mentioned having a hangover yesterday, so I deliberately worked the phrase into our conversation. Even writing about it now further reinforces it in my mind. I have found that vocabulary grows fastest when words move from passive recognition to active use. If you’re looking to expand your vocabulary, it is a habit worth cultivating.