Franklin Covey Co., trading as FranklinCovey and based in Salt Lake City, Utah, is a coaching company which provides training and assessment services in the areas of leadership, individual effectiveness, and business execution for organizations and individuals.
I'm probably biased, as a former instructor of a course built around this text, but this is one of the most comprehensive and easy-to-read style guides for business/technical communication that I've read.
Overall a good guide, and in Kindle form (via the reader in Chrome) it was easy to highlight and re-locate the bits most relevant to my needs. I especially appreciated the section on emails, as it had some good tips that were succinct and well-written.