The Elements of Style (Strunk & White)
A Guide to Writing Well (Fire & Knowledge)
Plain Language (The Federal Government)
The Copywriting 101 Series (Copyblogger)
Ten Mistakes Writers Don't See (but can easily fix when they do) (Pat Holt)
What is Good Writing  (Writing Center at Emory University)
Politics and the English Language (George Orwell)
Writing, Briefly (Paul Graham)