The Chronicle of Higher Education had a very popular article in their Review of Sept. 26, 2014 from Stephen Pinker, "Why Academics Stink at Writing." Following on this, they put together a free pamphlet on improving academic writing, Why Academics Stink at Writing -- and How to Fix it," that can be downloaded here. I do not think you need a subscription to the Chronicle to get the pamphlet. As far as I can tell, all anybody needs to do to get a copy is enter name, title, institution, and they get a PDF download.
Thank you! The image decorating this post is a mock-up of the cover of the pamphlet from the Chronicle.