Look for ways to find people who are already interested in your topic. This can be done offline if you think about it creatively. No matter what you are writing about, there are people who belong to groups, club, and organizations that are directly or indirectly related to what you know about. Visit these groups as your time allows and be sure to give out your business card. Your card should have you contact information and also an offer for a free article or report in exchange for their email address. This is a way for you to build a list of interested prospects before your ebook is finished. It will also position you as an expert within your area of expertise.