Written communication in business provides different writing challenges from academic writing (although there are some aspects that are similar). The sites and books listed here provide help whether writing a letter of application, writing a report, writing a memo, or writing any other business communication.
Business Writer's Free Library (Carter McNamara)
Workplace Writers (Purdue Online Writing Lab)
The Elements of International English Style: A Guide to Writing Correspondence, Reports, Technical Documents, and Internet Pages for a Global Audience by Edmond A. Weiss. “The book emphasizes principles of simplicity and clarity, proper etiquette, cultural sensitivity, appropriate layout and topography and more, and thus will increase the chances that texts prepared by native English speakers will be properly understood by non-native speakers” (back cover). (ISBN 978-0-7656-1572-5)