Especially when the users are almost zero
Since you asked, I wrote an ebook, MicroISV Sites that Sell! nearly 2 years ago that has garnered a lot of attention in the microISV community. http://www.47hats.com/ebooks/ (I was Inspired by your question to repost sample at my blog just now: http://www.47hats.com/2010/01/why-microisvs-fail-to-sell-2/ )
Check out the top section in BOS's FAQ It has a half dozen threads relating to your questions.
The first thread in the link lists several recommended books.
You can also check out a very good book for small companies/misv's The New Rules of Marketing and PR: How to Use Social Media, Blogs, News Releases, Online Video, and Viral Marketing to Reach Buyers Directly