There's several good blogs out there but I really suggest reading Traction [0]. It's well organized and very practical/easy to put into action. I've bought copies for multiple people (including my wife who recently joined a startup doing marketing).
Also you used the word "sell". Don't forget that sales is different than marketing. If you plan on doing real sales the book Predictable Revenue is great (though designed for slightly larger teams than just 2).
Good luck! And don't forget, NPS tools are good for brick and mortar businesses too.