Lanham, Richard A., Revising Prose (5th ed., Pearson Education, Inc., 2007), especially Lanham’s Introduction and Chapter 1 on self-diagnosis and revision, and pp. 64-67 on reasons why we don’t write well. The book’s foundation principle, often attributed to Supreme Court Justice Louis Brandeis, is that there is no such thing as good writing, only good rewriting.
McCloskey, Deirdre N., Economical Writing (2nd ed., Waveland Press, 2000) is a superb short essay on style in general and writing about quantitative analysis in particular.
Miller, Jane E., The Chicago Guide to Writing About Numbers (University of Chicago Press,2004). See especially Chapters 6 and 7 on creating effective tables and charts because writers for the policy development process should understand that tables and charts are important forms of written expression.
Purdue University, The Purdue On-line Writing Lab ( Purdue OWL), http://owl.english.purdue.edu This source includes an immensely rich catalog of short guidance items on issues of style, grammar, citation, and the writing process in general. Visit the website and look around.
Strunk Jr., William and E.B. White, The Elements of Style (3rd edition, Allyn and Bacon, 1979) is the classic collegiate guide to English prose composition. See especially White’s Introduction and Chapter V on “Style”, and Strunk’s Chapter III on “Rules of Composition.”
Truss, Lynn, Eats, Shoots & Leaves (Gotham Books, 2003) is a witty and readable essay on punctuation and its fundamental importance to clear expression.
Williams, Joseph M., Gregory G. Colomb, Style—The Basics of Clarity and Grace (4th ed., Pearson Education, Inc., 2012) is an excellent essay on the core principles of effective writing.