Purdue's Online Writing Resources
Although the focus is not public sector writing specifically, Purdue University has a great Online Writing Lab (OWL) offering free writing resources and instructional material. You’ll find everything from job search writing to all you need to know about use of semi colons.
The OWL Site Map provides a bird’s eye view of resources for writing and teaching writing; research; grammar and mechanics; style guides; english as a second language (ESL); and job search and professional writing.
All the material is copyrighted by The Writing Lab & The OWL at Purdue and Purdue University but individuals and teachers can use it.