I have been involved in book marketing, promotion and bulk sales since the mid-1980s, when my series of ten how-to business books were not selling as well as hoped/projected.  Since I was on a royalty contract with each of those books, I went to the publishers about me marketing my own books.  This was a new idea in the 80s – now it’s expected/required of authors being published by traditional publishers, and always required if you’re self-publishing your books.  Some people argue that PR is a subset of marketing; however, for books and authors, I find it useful to separate the two.
Marketing:  What I do is what marketers always do, for any product.  They use advertising and other mostly paid marketing techniques to get the book in front of potential buyers.  The closer the ad or marketing message is to the targeted audience, the more likely the books are to sell.  I have on my team an expert in social media marketing, which has become a serious player in the effective book marketing/promotion/ sales universe.  However, there are many other marketing programs available.  This varies based on subject matter, ans each market niche has its own media, some of which are open to advertising or joint ventures.
Promotion covers traditional PR as well as the many opportunities the Internet presents to named-authors to become more successful (i.e., to sell more books).  There’s an old saying in the marcom (marketing communications industry):  “Ads are what you pay for, PR is what you pray for.”  This means that PR “courts” platforms to allow articles (not ads) to be placed where target audiences will see them.  For authors, it works this way:
Being an author of a published book makes the named author automatically a topical expert especially in the eyes of the news media, and in some cases, the author is seen as a thought leader.  With the 24/7 news media now a pragmatic reality, the major news media is constantly looking for new talking-head experts who can put the subject into context for the audience.  Authors, by sharing their views in their books, are on-record regarding their “take” on the issue at hand, and are therefore automatically considered for on-camera, on-mic or in-print news media.  It’s just a matter of doing it right.
In 2008, we conducted a specific trial.  We focused on five under-reported issues in the Presidential campaign, and pitched our expert for those topics.  The results of five pitches were as follows:
On the air five times on Cavuto.

On the air five times on Imus

On the  air 56 times in radio interviews

In print or on Internet “print” more than 100 times during the campaign
This kind of result isn’t guaranteed, but we have duplicated these results many times for our clients. If your book makes you an expert, then we can make you a news media expert as well, which brings you and your book to the attention of potential book buyers.
Sales:  This is something few book marketers think about.  How to sell not one book, but 500 books in one closed sale.  We have proven results with both non-fiction and fiction books and authors.  For instance, Lawrence Block, a long-time murder mystery author and – for a decade, Writers Digest’s “fiction columnist” who later turned his monthly columns into several books with catchy titles like “Telling Lies for Fun and Profit.”  In one of his murder mystery series, he created the core character, a professional hitman who also likes to collect stamps.  Every new city he goes to in order to handle a contract, he stops by a local stamp store and finds some rare issue he needs to fill out his collection.
This became known to the national society of stamp collectors, or “philatelists” as they like to call themselves.  At one year’s national convention, they had Block as their keynote speaker. The room he presented in was standing-room only (SRO), and on each seat in the auditorium (at least 1,000) had a copy of Block’s latest Hitman-series book on the chair.  That represented a sale of 1,000 books along with his speaker’s fee.
After the talk, and later in the trade show, Block sat at the back and signed books.  Those who’d been standing (and didn’t have a book), books were for sale, and a convention staffer handled the sales, so Block could handle his meet-and-greet with people wanting him to sign his book.  That was after his talk.  The rest of the time, he was in the trade show with a booth/table. Again, a staffer handled the sale, while he handled the autographs.  All of his Hitman series of books – both novels and short stories – were available for purchase, and many people came by to purchase the whole set of them, and all of them was personally autographed by Block.  The hotel/convention center’s “business office” would, for a fee, pack and ship the books back to the purchaser’s home or office, a convenience because of the weight of books and all of the other things (stamps, stamp-collecting supplies, other books about stamp collecting) they wanted to acquire at the convention.
It was a big win for Lawrence Block, and it is one that any author has the potential to duplicate, based on the topic or distinctive feature of their books.  It works with novels, but it also works for non-fiction books.

While Block’s example is telling, I first learned how this works even for novels when I hosted (and researched) a one-hour program on “submarine disasters” for the History Channel, back when they actually cared about history.  In this were (among others) two novelists presented as submarine experts:  Tom Clancy, who’s break-out book, “Hunt for Red October” was all about modern submarine warfare; and Clive Cussler, who’s novel “Raise the Titanic” made use of “drone” submarines, long before they existed in the real world.  Between them, they’d sold over one billion books, which makes them big-time experts for the reading (and viewing) public.  These are personal examples, but there are many more novelists/subject matter experts who provide commentary on breaking news related to their expertise