I can't speak to "best," but I suggest reading Style: Lessons in Clarity and Grace by Joseph M. Williams, which crystallizes lots of non-trivial components of "good writing." (The link is to an older, less expensive edition which I used.)

I'll also second "write a lot" and "read a lot." Reading closely and with purpose in mind will speed up the latter (as opposed to the default of throwing books at your brain and hoping to pick up good writing by osmosis). Also, read good writers.