What's your goal? Presumably, to get people to buy your book. Millions of people, of course. But you can't do that all at once. So let's break it down to one customer at a time...
What causes someone to buy a book? In other words, what's the process, or the "sales cycle," as they say in other industries?
(Let's start with the assumption that you've written a quality book, and that the challenge now is to get people to know about it. If they don't know about it, they won't buy it, no matter how good it is.)
In my experience, there are several "channels" through which buyers take books to the cash register. Each of these channels has a series of discrete steps. You want to make sure that the steps are all provided for so you don't lose a customer en route.
Here's a simple route: I meet someone at a party, or the doctor's office, or on a plane... you get the idea, a stranger. When how I earn a living comes up, I say I'm a writer. Not a bad first step to the cash register, but we're not there yet. To help the person remember me and the books, I give her a card. The cover of the book is on the front; excerpts from reviews and my website URL are on the back. Now my claim to be a published writer comes across as much more credible and interesting. The quotes might provide fodder for further conversation. The cover is memorable. Maybe she'll even head straight for a bookstore, which would be great. More likely, life will interrupt the sales process. Until she's at work the next day, when the card falls out of her purse. Her supervisor isn't around, she's curious, she goes to the website, which is loaded with a lot more information, reviews, etc. than you can fit on the back of a card. "Wow," she's thinking, "these books sound great!" See how much closer we are to a buying decision now than we were at the party/doctor's office/airplane...? And of course, you include links to online booksellers right on your site so your potential customer becomes an actual customer via an impulse purchase...
One thing to ask yourself is, where can you find ready-made potential customers? I do a lot of mystery conventions. Doing panels gives me an opportunity to introduce pre-selected customers (mystery fans) to my books. Some of them will head straight to the bookstore after hearing me speak (God love 'em). Others will need to go through further steps, like the ones outlined above. Likewise for martial arts conventions. What about media outlets in your genre or market? Websites? Magazines? Etc? They have pre-selected potential customers, too. Other venues? Regardless of what channel we're talking about, remember to break the channel down into discreet "steps to the cash register."
As you go through these processes, you'll see that the tactics you use in different markets will overlap and reinforce one another. This is good. Usually it takes multiple "advertising impressions" to get a potential customer's attention, let alone to get him to decide to buy. He sees the ad in USA Today, then reads a glowing review elsewhere, then sees the book when he's browsing in the bookstore...
Ultimately you're looking for word of mouth, or "viral marketing," or "buzz." This is a related but distinct topic. One book I would recommend is The Tipping Point, by Malcolm Gladwell.
Let's break that last bookstore step down to further illustrate the "sales cycle." The potential customer sees the cover of the book first (previous advertising and your efforts at good shelf placement will have helped her notice it. How important is good shelf space? What steps do you have to take to get it?). Is she ready to buy now? Probably not. Probably we have to take her through some additional steps first. The cover is eye-catching, so she picks the book up off the shelf. She flips it over and looks at the back. There, she reads excerpts from several glowing reviews, maybe praise from another writer who she likes or has heard of. Now she wants to take the next step: the inside jacket, where she learns more about the book. It sounds interesting... okay, she wants to take the next step: The first page. The first page leads to the first chapter. By the time she's well into the first chapter, she's hooked. She's decided to buy the book, and heads to the register.
Of course, the sales cycle doesn't stop there. Because, again, the ultimate form of advertising is word of mouth. How can you get your new customer to tell other people about the book? Letting her correspond with you through your website is a good start. My attitude is, anyone who is enthusiastic enough about the books to contact me deserves a response and is likely to want to rave about the books to people he knows. Cultivate these people. They are your shock troops.
For everything I say here, stop and ask yourself how you can do it better. What more you can do. Whether any steps are being overlooked or neglected. All I can offer is an outline. You have to rigorously fill in the blanks and continually adjust your approach based on what you learn from your experiences.
I'm covering a lot of material here; there are many other channels, and you should break them all down into additional steps. For each step you identify, ask yourself: how can I strengthen this aspect of the sales cycle? You can't do it all alone. So who are your allies? How can you recruit those allies? Your publisher, by the way, is your most important ally. Hint: don't assume that because the people at your publisher have paid you an advance, they will back you as they should. You have to cultivate them, sell them, demonstrate to them that an investment in you will yield a higher return than an investment in someone else. This is a business. Learn to look at it from the publisher's perspective. If they're not giving you what you want, it's because you haven't done enough to show them that they should. Take responsibility. This is a winning attitude. It will make things happen for you.
Book tours: I've done four, each bigger than the last. They're certainly valuable opportunities to meet and sell directly to customers. But I would argue that they're even more important opportunities to meet booksellers face-to-face and build those critical relationships. Meeting the customers obviously is important, too, but a bookseller will have more ability and willingness to "amplify" the author's marketing efforts than the average customer. If I'm right about the value of meetings with booksellers, it follows that drop-in signings are almost as important as formal events. Plan your tour accordingly.
Here are two more books I would recommend on marketing etc. First, Selling the Dream, by Guy Kawasaki, which covers the fundamentals, but in a clear "Hey, I never thought of it in quite those terms" way. Second, Marketing High Technology, by Bill Davidow. Davidow distinguishes between marketing and brand as follows: if we're talking about a radio, marketing is the volume, brand is the frequency. I like that.
To me, when you talk about a brand, you're talking about the emotional connection a customer has with a product or service, the way the product or service makes the customer feel about herself. If using or being seen with the product fits with and enhances the image the customer wants for himself, he'll use more of it and pay more for it.
Books have brands and so do authors. In fact, anything that can be sold can have a brand. Jerry Seinfeld once said that he has a brand and for sure he does: hip, cool, funny, irreverent, in-the-know. Seinfeld's willingness to extend his brand to American Express was worth millions to AmEx. Regardless of what you're selling, it's important to think about what your brand is -- that is, how your product or service is going to make your customers feel about themselves when they use it. If you're conscious of your brand, you can take deliberate steps to foster it and avoid doing anything that would clash with it.
Packaging has a lot to do with a brand. So it's worth giving something thought to the way your title and cover art (which, in my mind, constitute a book's packaging) work together to forward your brand. What are the essential qualities of your book, the "hooks" that will most appeal to customers? And how are those qualities distilled and presented in the "look and feel" of the book's packaging?
Okay, two more book recommendations. Although in some ways the authors take their arguments too far and in others don't take them far enough, their points are worth thinking about. First, The 22 Immutable Laws of Branding. Second (inevitably), The 22 Immutable Laws of Marketing.