Anything and everything. Good books are encouraged, excellent books are a must, and terrible books are also useful, in the same way all mistakes are useful— they can be learned from. As a writer, reading (and understanding) the mistakes of others will keep you from making those same mistakes yourself.
‘Excellent books’ goes without explanation— humans are copycats from the minute they are born. Just as we learn how to talk by listening, we learn how to write by reading. The more we listen, the more we find things of our own to say with the words we hear. And the more we read, the more we discover our own voice by studying the voices of others.
All the grammar and English I ever studied went in one ear and out the other, but because I’ve made a habit of reading excellent books, I know what beautiful words look like, which is the first step to being able to wield them.
So go forth and read. The world is at your fingertips.