Reddit reviews On Writing (Modern Library)

The best way to improve your writing is to read. Honestly. It sounds simple but it's true. If you read and read and read you get better. You see what you like, what you don't. What works for you and what doesn't. Really though, more reading=better writing. Also, write all the time. Constantly write. Even if you this it's shit, just write. Not everything you write has to be good.
Also, these two books have helped immensely and I've read them over and over again. They're good for understanding how "writing" works.

Borges, On Writing

Eudora Welty, On Writing