To become a writer, you will have to learn how to write. This may seem obvious, but many people think they know how to write because they graduated from high school or because they read a lot (this is an attitude, by the way, that infuriates most professional writers). When I was just starting as a writer, I thought I thought I knew what I was doing. After all, I had made A's in English class and I knew I had some natural ability. Now I know that good writing means a whole lot more than just grammar or punctuation. Developing writing skills takes a lifetime. John McPhee said, "The thing about writers is that, with very few exceptions, they grow slowly—very slowly."
There are some wonderful books out there to help you become a writer. These are my favorites:
Bird by Bird by Anne Lamott
Warm and wise guide to writing. Exceptional advice on writing first drafts and taking a moral stance in your writing.
The Elements of Style by William Strunk and E.B. White
A classic packed with advice on sentence construction, punctuation, word usage, and more. An education in under a hundred pages.
The Giblin Guide to Children's Books by James Cross Giblin
A no-nonsense guide to writing books for children full of useful advice. The chapters on writing nonfiction are excellent.
On Writing by Stephen King
A funny and moving look at what it means to be a writer, along with nuts and bolts advice for aspiring writers.
On Writing Well by William Zinsser
Zinsser tells you many rules of good writing that you wish everyone—professional writer or not—would follow.
Queries & Submissions by Thomas Clark
The single most helpful book I've read on how to research and query magazines. It is out of print, so you may have to dig a little to find a copy.
Writing for Story by Jon Franklin
This book analyzes the structure of a dramatic nonfiction story and guides you step–by–step through researching and writing your own.
Writing Picture Books by Ann Whitford Paul
If you dream of writing picture books, this is THE book to read. Chapter by chapter, you'll analyze current picture books as you write your own.