You can just read anything, it doesn't have to be a book. I focus on words that get the point across, not just synonyms to replace the simple ones. Just don't end up like those fools trying to use big words IRL to sound smart, that never comes off as genuine. However, on the internet you have time to look up words and use them in your posts to sound smarter. No one can prove you are a dumbass over the internet if you know how to use google.

The only time I type on a college graduate level is when a paper requires it. I use the Flesch-Kincaid readability tests to determine the reading grade level of papers I'm working on. In the end, imo, simplicity isn't always equal to stupidity, just focus on conveying your ideas on a level that will be easily understood by most on a first read, you don't need big words to do that.
edit: Focus on punctuation, structure, and grammar too, those are very important aspects of writing that also transfer over to speaking.
I read books all the time my vocab hasn't gone up. I read mostly fiction books tho like RL Steine, Judy Blume , etc etc
Do it have to be non fiction books?
Because it's not just about reading, people think you can just read and have larger vocab. You need to understand what you are reading first. IMO, people with English as their first language have the worst understanding of the language they speak, this also applies to any other first language. What is the first things you work on when learning a new language? You work on grammar, pronunciation, etc. You leave all that behind at gradeschool and unless you are attending a school with a good curriculum, your writing and English level either declines or becomes stagnant over time.