How to Write Short: Word Craft for Fast Times by Roy Peter Clark
Grammar Girl’s Quick and Dirty Tips for Better Writing by Mignon Fogarty (various books, app, podcasts)
Nicely Said: Writing for the Web with Style and Purpose by Nicole Fenton, Kate Kiefer Lee
Content Strategy for the Web, Second Edition by Kristina Halvorson, Melissa Rach
Oxford Guide to Plain English, Fourth Edition by Martin Cutts
Hemingway App – Makes your writing bold and clear (desktop, mobile versions)
If you have any other suggestions, please send email to firstname.lastname@example.org.