Writing better
Books commenters say made them better writers.
Books several people mention
Ranked by how many different people say it helped. Small numbers, so the order is a hint.

A guide to clear, effective nonfiction writing, covering technique, jargon, and writing blocks.
I feel that it helped me learn to write better business emails, by getting me to focus on writing clearly and concisely.

A guide to revising prose so it reads clearly and gracefully for readers.
I almost can’t overstate how much it’s changed how I read and write. Before that book, some writing just felt “clear” and other writing didn’t, but I couldn’t explain why.

A short classic manual of English usage and writing style rules.
I, myself, have begun more thoroughly documenting projects, not only for my sake but for everyone else that comes along. I find that this book helped me get up to speed quickly

How the internet changes language, from texting punctuation to memes and slang.
It actually has changed how I write on slack. I'm now for example more deliberate about omitting or including periods, break up what before would have been a long, email-like message into smaller messages.

A memoir of the author's life combined with practical advice on the craft of writing.
Two books that have helped me improve are Stephen King's On Writing and William Zinsser's On Writing Well.