My first customer found me before my showHN post ( publicsonar.com - they needed a geoparsing tool). I'm not entirely sure how they found me, perhaps from a talk I gave on my book scanning project.
I also don't know how the other early customers found out, I'm not doing any marketing (other than conference presentations).
Can you link that book scanning project? It may be a blog post I read on HN and really liked but subsequently lost the ability to find.