On a Linkedin forum for writers, B.C.B. said, "I just need help deciding where and how to advertise online."
My response (slightly modified here):
For a nonfiction book, Google's AdWords can be effective and efficient, appealing to potential customers searching for such topics as "Thomas Jefferson" or "beer brewing." You want to appeal to people interested in a topic, even if they're not necessarily shoppping for a book.
OTOH, advertising fiction would probably be extremely ineffective and inefficient if you try topic-based ads for "novel," "crime novel" or "teenage vampire sex."
Google's AdSense is more expensive than AdWords, but can put bigger ads in front of bigger audiences. It places ads on many websites, but not on the Google search site. It can display ads (1) related to the content of a page, (2) the demographics, market segment, geographic location, or URL, (3) website users' interests and previous interactions with an advertiser.
You can also negotiate to pay for ads on specific websites and blogs which are likely to attract potential customers for your book. If you have your own website or a blog or forum related to your book subject, naturally they should advertise your books. I do this with my books about telecommunications.
Book ads that are inserted on webpages without being inserted because of searching or site visits or some other characteristic (i.e., not AdSense or similar ads) could quickly bankrupt you. Big companies with a wide variety of customers can advertise on lots of websites. Citibank advertises on the Time magazine site, but you probably can't afford to. You probably can't advertise on the MTV site like Chevrolet does, either.
Concentrate on press releases, reviews and online media. Get your name and book into comments on lots of blogs and news websites. Even if you are commenting on an article about Santorum or pizza on the NY Times website, your "signature" should mention and briefly describe your book. E.g., "author of Failed Ford -- the story of the Edsel."
You can create a press release or an online comment to help you "hitchhike" on a popular author's book or a related topic, so people searching for "Sarah Palin" or "senior prom" may find you.
The web has robots that search for key words and phrases and may provide added exposure for you and your book. One of my books was mentioned in "Japan Diaper News" because a reviewer said it is so funny that it will make you pee in your pants. Another robotic website specializing in Cuban politics mentioned one of my books because of a casual mention of the Bay of Pigs in something I wrote. Even irrelevant websites can improve your visibility and the ranking of your own website or blog.
Word-of-mouth is very important. Sometimes it seems like I sell one book at a time, but that one sale can lead to many more (for a good book).
Be prepared to give away at least several dozen books -- both at the beginning and over the life of the book. These are not just for book reviewers, but for INFLUENCERS with whom you share some bond either personally or because of the book, and who might recommend your book online or on radio or TV.
If you can't motivate a talk radio superstar or his or her producer, try pitching the second banana or call screener who may influence the boss.
Look for reasons to promote your book based on the calendar or appropriate news events such as a war, anniversary, holiday, divorce or death.
Mention your book in everything you write online, and to anyone you meet in "real life" who might become a reader -- even people on a supermarket line near you or next to you on an airplane. Listen to other people's conversations or notice what they are carrying or buying to provide a reason to talk. If there is no clue, you can always start with the weather. A smile will help.
Write reviews of books that appeal to your audience -- and mention your book.
Always have business cards with you that show your book title and tell where it's available. I insert about six cards in every book I send out. I get them from VistaPrint.com. I also insert cards in packages that my phone equipment company sends out.
An author can't afford to be timid. If you're reluctant to toot your own horn, you'll have to hire someone to toot for you -- and hired tooters may not understand your book or share your passion.
My Stories I'd Tell My Children (but maybe not until they're adults) has sold over 2,000 copies. That's pretty amazing for a memoir by a non-celebrity, but it takes a lot of work to sell that many.