Oh, and if you're interested in the subject, I can also recommend "Traction" by Gabriel Weinberg and Justin Mares, it's a really neat outline of traction channels.
I also really enjoyed Traction. It provides a good framework on testing to identify the channel that provides the results needed at the stage your business is at. It also goes drives home the concept that the channel that gets you to one stage of growth might not be the one that gets you to the next stage.