As plans are being made for a book to be published, careful consideration needs to be made about how to market it. The decisions can fall to publishers or authors themselves, sometimes with input from trusted business contacts like literary agents. Big-budget releases may include a publicity and advertising program. Many others may choose one or the other, and customarily it would be publicity. Advertising is paid media that you control, and publicity is earned media coverage that costs nothing. Most industry wisdom and years of experience have shown that free media coverage gives books more credibility.
While there is nothing wrong with paid ads, they customarily require a substantial budget and, unless they reach a critical mass point, may not be seen by enough potential readers. On the other hand, media coverage can be strategically placed and have an excellent chance of reaching target readers. It's also an axiom that audience size doesn't always equal results. For example, podcasts are having exponential growth and have become excellent publicity vehicles. Some have smaller niche audiences, but those listeners can be highly interested in a book's topic and much more prone to buy it than a large general audience.
Publicity also has invisibility to many people because when books and their authors are covered in the news, it's natural to find out about them. Finding out about something makes a prospective reader feel knowledgeable, and it's from the media they respect. It's a different experience from viewing a paid ad and likely much more influential. Authors also need to keep in mind that media coverage isn't controlled, so the outcome is not guaranteed. But most coverage is beneficial to a book, and if you work with a professional publicist, they can approach media with a good track record of positive coverage.
Industry experts also advise it's hard to earn a favorable return on investment with book advertising. It can be helpful for branding exercises but not as much when tied to sales. An author's platform – social media, website, the professional network for word-of-mouth – along with media coverage are generally much more helpful. Because most books today are sold online, it's also crucial to complete book and author pages thoroughly with searchable keywords on major bookselling websites. There isn't a silver bullet for sparking book sales except for going viral on the internet, which takes a fluke and incredible good luck.