If you are going for the textbook look, you got it with both the title and the cover. Who is your audience? Is it home gardeners or students? The word sustainable has been so overused. I'm not even sure most people understand what it means. I would suggest you simplify--say exactly what you mean and to invite people in to read the book put a glorious picture of the end result. Think about "Working with Mother Nature" a guide to a sustainable garden, or some such.