I sold my first book nearly five years ago. I'm asked on a fairly regular basis by aspiring writers to judge an idea they have for a book. Or sometimes they want the inside track on how to get published. Sorry, but there is no inside track. I don't have it, and no other author does. But I can share what I've learned in my short time in the business.
1. There's a lot of luck involved. The right book to the right editor/agent at the right time can make all the difference. There is hard work and talent involved in making something like the Harry Potter series or Twilight a phenomenon, but luck plays a huge part, too. Making millions in this industry (or hundreds of millions) is the equivalent of hitting the Powerball lottery.
2. Ignore writing books. That's right. Keep walking past the writing reference section in your local Borders. With the exception of Writer's Market and a good style/grammar guide, leave them alone. Why shouldn't you buy the latest writing book that tells you how to write a megabestseller? Read too many books on writing and you start to suffer from what golfers call "paralysis by analysis". Outline. No, don't outline. Never open a book with weather. Never write on Tuesday during a solar eclipse. Pretty soon instead of writing you're staring at a blank page, frozen. The best writing books are the novels you have on your bookshelf. Read them. Learn from them. Figure out what the author did and how he did it.
3. The world at large doesn't give a shit that you wrote a book. Store clerks will be indifferent. Customers in a bookstore will get whiplash trying not to make eye contact with an author holding a booksigning. Must not look. Will be forced to speak with author. And as for book signings, they are mostly a waste of time. May the self-promotion gods not strike me down for that last statement. Unless you are a well known author with a large following (or can bribe your friends and family into coming), you'll probably spend most of the time at a table. Waiting. That's not to say you shouldn't promote. Write a blog. Meet other writers. Get online and participate in forums. Put excerpts and free stories on your website.
4. In the name of all that is holy, don't get in it for the money. For a first time, mass market paperback deal, you can expect between $2500 and $5000 for an advance. And around an 8% royalty rate. Write because you can't stop telling stories. Because you're always composing something in your head, turning things into plots, playing "what if." Keep writing because you love it, and if you make money, great. If not, be glad you've been blessed with a talent and use it.
5. Don't quit. There'll be rejection. Lots of it. Even after you're published. Keep writing. Always.
Take the above advice for what it's worth. Your mileage may vary.