Book:
The McGraw-Hill Handbook of Business Letters (Third Edition)
http://www.amazon.com/McGraw-Hill-Handbook-Business-Letters/dp/0070504512/ref=sr_1_12?ie=UTF8&s=books&qid=1221858646&sr=8-12
Newsletter:
Business Writing Newsletter - Focused on Important Writing Issues
http://www.businesswritingsolutions.com/newsletter/index.php
Blog:
Business Writing
http://www.businesswritingblog.com/business_writing/
Discussion Forum:
Business Letters:
http://www.englishforums.com/open/Business+letters.htm
Letter Writing:
http://www.usingenglish.com/forum/letter-writing/
Peer-Reviewed Journal:
Journal of Business Communication:
http://job.sagepub.com/
Subscription Information:
http://www.sagepub.com/journalsSubscribe.nav?prodId=Journal201671
.edu:
The Owl at Purdue - Writing the Basic Business Letter
http://owl.english.purdue.edu/owl/resource/653/01/
The Owl at Purdue - Sample Letters
http://owl.english.purdue.edu/owl/resource/653/02/
Business Letters - Help in writing letters for different situations
http://www.unc.edu/depts/wcweb/handouts/business.html
.com:
Business Letter Format Tips
http://www.writeexpress.com/business-letter-format.html
Library of Sample Business Letters
http://www.partafzar.com/business_letters/index.php
Letter Writing Guide:
http://www.letterwritingguide.com/
Wikipedia
Business Letter:
http://en.wikipedia.org/wiki/Business_letter
Link Exchange:
The Owl at Purdue - Writing the Basic Business Letter
http://owl.english.purdue.edu/owl/resource/653/01/