Nicely Said: Writing for the Web with Style and Purpose by Nicole Fenton and Kate Kiefer Lee
Content Everywhere: Strategy and Structure for Future-Ready Content by Sara Wachter-Boettcher
Content Strategy for the Web by Kristina Halvorson and Melissa Rach
The Elements of Style, Fourth Edition by William Strunk Jr. and E. B. White
Blog Posts & Articles
Five Steps to Plain Language from the Center for Plain Language
Federal plain language guidelines from plainlanguage.gov
UX Writing: How to do it like Google with this powerful checklist by Guy Ligertwood
Writing for UX: some practical tips by Anjana Menon
The art of the error message by Marina Posniak
16 Rules of Effective UX Writing by Nick Babich
Why Your Links Should Never Say "Click Here" by Anthony T.
Content Guidelines & Other Resources
Hemingway Editor identifies problematic language such as passive voice and adverbs
18F Content Guide: How 18F plans, writes, and manages content
gov.uk Content Design Guidance: Planning and managing digital content to meet the needs the public has of government.