Clean Code by Robert Martin. You'll see it mentioned frequently on HN and online in general. Changed the way I wrote code almost immediately. I kind of wish it had been a part of my university curriculum. I'm sure we HNers can pick on the book for this and for that (can't any book?) but overall I think it's excellent.

For other books, this page is also a good start: https://hackernewsbooks.com/top-books-on-hacker-news