You have to read to be a good writer, read constantly. Authors you love, envy, even those who you dislike but have got a certain style or knack of doing something right (Hemingway falls into this category for me). I find that I can't read a lot of fiction when I am writing first drafts, but I read as much as I can before and after.
You can also read to learn to improve your writing. There are lots of books for writers out there that can all be helpful in their own ways, you just need to decide what style you need.
Warning: don't spend all your time reading how-to-write books instead of writing. These books are meant to supplement your writing, inspire it and help improve it. Reading all the books in the world won't help you write if you don't pick up a pen or sit at the laptop (and not surf FB).
I fall between liking books that giving me lots of writing prompts and those that are about specific craft problems. I struggle with plots, so often seek out books specifically on that topic. But some days I just need a kick up the bum and then I will dip into some old favourites to get me going again.
One book that really inspired and helped me early on was Writing Down the Bones by Natalie Goldberg. Published in 1986 it has become a creative writing bible. It promotes the idea of writing exercises and writing everyday which really clicked with me. Mixed with references to meditation and Zen Buddhism it is a bit new ageish, but they are not overwhelming. Natalie's main focus is to get you writing. I've read some of her follow-up but they didn't really push me in a way that Bones did.
Anne Lamott's Bird by Bird, 1994, was a good follow-up to Goldberg's book, dealing with some of the troubles that pop up when you make writing a daily habit. Her religious undertones were a bit much for me, but on a practical side the book is very helpful.
A book focusing on poetry that really struck a chord with me was The Triggering Town, 1979 by Richard Hugo. I was living in the Pacific Northwest at the time, Hugo's stomping ground, and I could see so much of my surroundings in Hugo's work, but also I loved his idea of a triggering town, using a subject or word as a trigger to start off the reader's journey into the world of the poem, real or imagined, focusing on the language and its music once they are drawn in. Hugo's gentle self-effacing tone was a lovely introduction of workshopping and owning your poems for me.
I think I have previously mentioned A Writer's Book of Days, 1999, by Judy Reeves which has hundreds of writing prompts to help you find inspiration for your daily writing exercises or for when writer's block hits.
On the fiction side I have turned back many times to The Art of Fiction by John Gardner, 1991, based on his lecturers to beginning creative writing students over the years. It contains some analysis, some practical exercises and lots of things to look out for.
There are also practical books like Elements of Style by William Strunk Jr and E B White published first in 1918 which discusses grammar and usage rules for writing in English. Dry, but it's a good reference for editing.
There are very technical books like Mary Oliver's A Poetry Handbook, 1994 which breaks down the elements of poetry with lots of examples of styles. It's great for getting started but if you've studied literature or been writing poetry for a while it may feel very basic.
There are lots of good books I haven't read. I keep meaning to pick up Stephen King's 2000 memoir On Writing. I've heard it mentioned many places as inspiring and helpful, but I've never had the chance to pick it up. Here's a hint of his rules for writing to get us all started.
It's best to dip in and out of these books rather than spend lots of time reading them. I often try the writing exercises or editing ideas out as I read, so that they just don't become filler.
Any suggestions for new books or old favourites always welcomed here.