A Plain English Handbook: How to create clear SEC disclosure documents

Published by the US Securities and Exchange Commission (SEC), this handbook shows ‘how you can use well-established techniques for writing in plain English to create clearer and more informative disclosure documents’. 
http://www.sec.gov/pdf/handbook.pdf

Bartleby

Publishes several classic books on line (in full), including The Elements of Style by Strunk and White, and The King’s English by Fowler. 
http://www.bartleby.com/usage/

British National Corpus (BNC)

A corpus is a collection of samples of written and spoken language. Type in the word or phrase you’re interested in, and you’ll get a list of instances headed by a note of the total frequency of the word or phrase – useful for deciding which word or phrase is more familiar and so better to use in your document.
http://www.natcorp.ox.ac.uk/

BusinessDictionary.com

Featuring over 20,000 terms and over 115,000 links between related terms, this website aims to provide ‘a clear and concise description of any and all business terms’. 
http://www.businessdictionary.com/