Much will depend upon the market. Every book has a limited appeal, the trick is finding those to whom it appeals. Direct mail does allow gut reactions to be tested, again and again. Small test mailings to groups of people who may be interested. Responses will come from unexpected quarters, this is part of the fun, but be prepared to recognise and accommodate these new opportunities as they arise.
Present contacts Existing customers and contacts are the best place to start.
As you meet new people they should register as potential buyers. Read articles with that in mind, noting interesting names. All correspondence should be examined from this viewpoint. A simple database will suffice, include any information that will help build a picture of the ideal customer. Non-computer users can use a card index.