Eliot Coleman is a fellow Mainer. He lives downeast from where we used to live. He grows greens all winter long and sells them at local restaurants and grocery stores. And he doesn't use a heated greenhouse! His book outlines this wonderful system of cold frames, succession planting and cold hardy plants that he's developed. It enables you to harvest fresh greens all winter long. Now granted it's not like you're picking baby bibb lettuce and cherry tomatoes, more like kale and mache and wintered over carrots. The carrots he calls "candy carrots" because apparently there's some sort of gardener's alchemy that occurs in the sugars when they winter over and it makes them extra sweet.
The book is a really enjoyable read, it's like talking to your expert gardener neighbor over the fence. He adds little asides and tells you about his favorite plants, tools and tips. Mostly I like that his methods are pretty simple. He's not about making more work for yourself just for the sake of getting fancy. And the methods are tried and tested. If he can grow greens in the harsh Maine winters, you can grow greens anywhere!