Zinsser is one of my favorite writers about writing. Many people swear by Strunk & White but I love reading Zinsser and have found his advice invaluable. He's written lots of great books but the way I refer to over and over is On Writing Well. His chapters on simplicity and clutter are classics, he sees all the pitfalls of academic writing (which I come up against in my editing work again and again), and although the book was first published in 1976, it remains fresh and to the point.
A few hours spent with Zinsser are an excellent and very cheap way to improve your writing. Worth more than many writing workshops.