Marketing a book means, rather obviously, getting people to read it. Novels are sold by word-of-mouth and good reviews, but social media has changed how this happens.

As part of a marketing strategy developed from the moment a book is acquired, publishers send galleys or advance review copies to reviewers and booksellers. Advance copies generate blurbs for the final cover and also early interest in the book via print reviews, author interviews and spots on television and radio.

Nowadays, a strategy of digital advertisement is thrown in. Publishers and authors need to tweet, Instagram, post on Facebook and connect with readers online to keep the buzz about their books alive, say marketers from HarperCollins and Macmillan Publishers International. Social media marketing is as important as traditional forms of publicity.

"Eight years ago, for a big book or series, a marketer would book a big, expensive print ad in the New York Times or People magazine," says Ms Megan Traynor, assistant marketing manager for international Sales at HarperCollins Publishers. "Now, it's a lot less expensive to launch a book onto a bestseller list. Social media is crucial for most of our campaigns."

Consider HarperCollins' young adult fantasy series by Victoria Aveyard. The first book, Red Queen, came out in February last year and hit the New York Times' bestseller list in the first week after its launch. Apart from being a record of good sales, a spot on such bestseller lists also acts as advertising.

Six months later, HarperCollins began pushing the still-to-be- published sequel to Red Queen on social media channels as well as its website EpicReads, which targets fans of young adult fiction.

In June last year, the title of the sequel was revealed: Glass Sword. In July, the cover.

In succeeding monthly updates, new novellas in the same universe were announced for later sale in e-reader bundles and in paperback. A book trailer for the sequel came in November. In December came sneak peeks online of the first three chapters of Glass Sword - with helpful links for pre-ordering the book - and a post on EpicReads suggesting Christmas gifts that would suit Red Queen fans.

In the weeks leading to the launch of Glass Sword on Feb 9, there was a book trailer, posts sharing fan-made artwork that celebrate the Red Queen universe and even more sneak peeks of chapters - with more reminders to pre-order.

"The emphasis has become more and more digital," says Ms Traynor. "It's always great when an author has an existing social media platform, but when they don't, marketing will try to build their profile."

It is the same for Pan Macmillan marketers Sarah Mclean, head of international sales - open markets, and Naomi Shields, international sales and marketing executive.

To push their titles, publishers have official accounts on social media platforms such as Twitter, Facebook and Instagram. Their YouTube channels feature author interviews, book trailers and reviews.

Marketers also connect with review sites where readers congregate, such as Goodreads and Amazon.

Here they offer sneak peeks of content as well as exclusive interviews with authors, and set up online chats between fans and authors.

The ease of publicising a book internationally online has led to some changes in marketing, publishers say.

For example, HarperCollins will release Singapore-born Cheryl Lu-Lien Tan's first novel Sarong Party Girls in July.

In the lead-up to the launch, marketing efforts are likely to focus first on Singapore and South-east Asia as important target audiences. Previously, markets outside the United States and United Kingdom often received new titles well after the American or British launches.

Last year, however, Macmillan thriller Luckiest Girl Alive by Jessica Knoll was released internationally four to six months before the British edition came out. International markets were ready, judging by the buzz.

Before creating online buzz, marketers seek reactions in-house. They ask colleagues to read the book and share their thoughts. These are used as blurbs on advance review copies and posted on social media. Macmillan's BookBreak series of videos, for example, has book-loving employees talking about the titles they love and comparing covers and characters.

Macmillan's Ms Shields says: "You can't get people to fake their enthusiasm. Readers want a genuine response to the title."

Advance review copies traditionally have a different cover from the final product and are often very striking, in order to pique the interest of a reviewer or bookseller.

With the popularity of image- sharing, the cover design of a book has taken on new significance.

Hanya Yanagihara's A Little Life, a heart-breaking tale of four friends in New York City which was shortlisted for last year's Man Booker Prize, has a man's face on the cover of the Doubleday edition. Fans on Instagram now regularly post selfies with the book cover replacing their own faces, images that the author shares on her official account.

There is also geat demand for the marketing materials launched to promote A Little Life, including tote bags featuring the names of the central characters. Tote bags or mugs with quotes from the new book are often offered free when a novel is launched.

With these new avenues of garnering publicity, a marketer's job has, in some ways, become harder.

"With social media, there's a lot of noise about a lot of books. Marketers have to find a way to be heard," says Ms Traynor of HarperCollins. "You still have to have a good message."

And, of course, a good read.