There are four books at arm's length in my office:
Strunk and White's The Elements of Style is still a good, if not perfect, reference on style. Young writers should calibrate their reading of Elements in light of criticism from linguistic experts. Experts claim that the good parts of Strunk and White are common sense. I take issue only with their application of the modifier common. My counterparts in linguistics and the humanities must have never seen the aggressive absence of style that permeates writing in science, engineering and mathematics.
After reading this post, my colleague Zachary Tatlock recommended Style: The Basics of Clarity and Grace as a better reference than Strunk and White. Having had a chance to read it, I agree with Zachary. If you can get only one book, this is the one to get. (I'd still recommend a thoughtful reading of both.)