Some books about writing are great, this is one of them. With one part memoir (as in "this is how I came to write") and the rest about the craft of writing (anything from dialogue to storytelling and what not to do) On Writing is a must read for any writer or future writer.
ps: My favorite quote is this one: "The sort of strenuous reading and writing program I advocate—four to six hours a day, every day—will not seem strenuous if you really enjoy doing these things and have an aptitude for them"