Clear English
Good Writing Skills
HarperCollins 0-00-717293-1
Very crisp and packed with ideas. Good fun too. Part of the Collins Wordpower series.
The Complete Plain Words
Penguin Reference 0-14-051199-7
Third edition by Sidney Greenbaum and Janet Whitcut. The classic Civil Service guide to clear writing. Needs application, but is well worth the effort. Serious, but not stuffy.
Gobbledygook
Atlantic Books 1-84354-356-7
Scathing attack on business and management language. You will be loath to use strategy, implementation, facilitation, initiative, enhancement, value-added, flexibility, outcome and commitment again. Death to all mission statements.
Indlish
Viva Books 81-309-0281-8
Excellent, clear, vigorous advice about how to write clearly. It's particularly helpful for Indian writers who need to get to grips with the different syntax of English, but native writers of English will also enjoy and benefit from it. My review of Indlish for the magazine of the legal language group Clarity (Clarity, Number 58, November 2007) is available here.