What kind of books do you read to improve your writing skills?

William Faulkner answered your question along time ago, and I quote him '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.' 

And I believe your readings should rely more on what is the subject you desire to write about, that will help you better. 

i read novels,articles,and gain knowledge by watching documentries.

I read books in the range of weapon/warfare books/encyclopedias along with articles from old magazines such as Soldier of Fortune. I also read Tom Clancy novels; also about warfare. This improves my writing skills by showing me how to keep my writing to the point and bring main items to focus. 

I read a wide range of genre. This helps me broaden my knowledge.