About a year ago Seth Godin published Who’s There? — a free ebook about the evolving art of marketing with blogs. (This was a follow-up to his previous ebook Knock Knock which discussed how to make websites do their job better.)
This 45-page PDF is a quick read:
Seth Godin’s Who’s There (PDF)
Seth’s book provides a good introduction into the world of using blogging as an effective marketing tool.