Equity Avenues
Angel Investors & Incubators
Note that there are private investor groups in most major markets. They meet regularly and discuss opportunities in their city. Investment bankers make it their job to know these kinds of people.
Venture Capitalists: Only 1% of deals they see get funded. VC's don't care for book publishing. Better have a great story.
Private stock offerings: Many laws have been passed that make a private offering more attractive and easier to do.