If your goal is to sell books, you must view your book as a business. In what ways do you treat your book as a business? Where could you improve? What resources could you leverage to improve your book business?
I'm making this one quick because I have a lot of work to catch up with this week. Talk about writing as a business! You learn to appreciate writing in a much different way when it determines whether or not you pay your bills or eat in a given week.
Anyway, the most important part of the whole book-writing business...is finishing the damn book. You can't market much if you don't have a product to sell. And that counts for subsequent books, too. You have to keep writing or else you don't have anything to sell.
Past that, I think marketing is a combination of making friends and using your creativity to get people talking about your book. Which, of course, only works if you have a good book in the first place.
Oh, and one piece of advice before I get to work today: Don't take things so personally. Don't be afraid of failing. Just try things out. Experiment. If something doesn't work, well, you might have to go back to the drawing board. But I think the second step of running a business (after you create the product that you're selling) is to be flexible in your approach and always find solutions instead of just balking at problems.