In my humble opinion, the best writing advice ever comes from American writer and Nobel Prize laureate, William Faulkner—
"Read, read, read. Read everything—trash, classics, good and bad, and see how they do it. Just like a carpenter who works as an apprentice and studies the master. Read! You'll absorb it. Then write. If it's good, you'll find out. If it's not, throw it out of the window."
Here are a few reasons behind my opinion.
You can see how they do it
As a writer, reading books from one's own genre helps you to master the formula of the genre. Every mystery or thriller fan opens a book with certain expectations. That's because crime fiction novels are highly formula driven. Whether we're speaking of Dashiell Hammett, Raymond Chandler, or Robert B. Parker, while each had his own unique writing style, all of them followed the same "rules" when crafting their novels.
Crime fiction authors today who hope to be successful must also follow those same rules. Reading crime fiction books allows you to learn the rules, master the formula, by seeing how the great crime fiction authors of the past did it. You can learn more about character development, plotting, and writing dialogue by reading crime fiction novels than you would learn from reading a hundred "How to" books on writing.
Just like William Faulkner put it, while reading "you'll absorb" the process of writing, and that will make you a better writer.
As an apprentice and studies the master
There are even more benefits from reading than simply learning the formula and rules of your genre. Faulkner advised: "Read everything—trash, classics, good and bad..." which means don't limit yourself to just reading genre specific novels.
Read books from other genres, and especially don't neglect the classics. When I say classics, I'm speaking of the great novels of the past you will find on any of the many "100 books to read before you die" lists. Books like The Great Gatsby, To Kill a Mockingbird, Lord of the Rings, Brave New World, etc. Reading the classics will enrich you as a writer in a myriad of ways, but one important benefit is that it will expand your vocabulary.
The tools to write
Stephen King, another author who knows a thing or two about good writing puts it quite bluntly in the above quote: "If you don't have the time to read, you don't have the time (or the tools) to write." It's through reading that you acquire the tools to become a good writer.
Personally, I've been a bookworm since childhood so finding the time to read has always been a priority for me. In fact, my love of reading probably has more to do with me desiring to write and publish books than anything else. But occasionally other authors tell me that they simply can't spare the time to read. They are just too busy. If that describes you, better reconsider my friend. If you aspire to be a successful author, better heed Stephen King's good advice.
Read as much and as often as you can. And always remember, every good writer must be a reader first.