This manual is used by most departments of the Australian Government and by many other organisations, even though some departments also have an additional style manual for their department.
The style manual covers the planning process, writing and editing, design, illustration usage, the legal aspects of publishing, and final product evaluation.
I have found the writing and editing section most useful and recommend it for both writers and editors. In particular, there are in-depth sections on grammar, spelling, punctuation, capital letters, shortened forms, citations and references.
The manual is available online from Booktopia.
To find other recommended resources for writers and editors, check the resources page.