It can depend on the type of book and goal for your book. But the strategies I know of working and that have worked for me. Involved building an audience of your own and leveraging other peoples audiences.
So if you don't have an audience go through and find the thought leaders and personalities in that space. Chat with them about the book and see if you can get them on your launch team. Start Sending out advanced copies to these people to get their feedback and support. Then you want to create some pre-sale freebies. With Mark Mason The Subtle Art of Not Giving a Fuck. Had a bunch of freebies if you pre-orded it for 20 bucks or something. He also appeared on a shit load of podcasts and had a bunch of people promoting his pre-launch.
I have tried a variety of methods and found that ONLY two methods work to generate interest and sales if you are starting FROM SCRATCH with NO platform, NO fans, NO influencer contacts.
I will focus on pre-orders at Amazon.com. The most important Section of the Product page is 'Customers who Bought this Also Bought'. The only way for your books to get visibility (and thereby sales) is by getting your book to show up on this Section of related books in your genre.
1. Facebook ads.
You will need to create a Click to Websites Link ad campaign. When you click on Create Ad in Facebook, this is the option you should select.
Next, you will create an Adset. Very Important: In the Audience Targeting Interests section, type in the name of authors only.
Pro Tip: Use the Narrow Audience to further filter your audience. Ex. if your book is a children's middle grade fiction, you can use first audience as J.K. Rowling and then select Narrow option and then second audience Rick Riordan.
The final audience size should ideally be around 100k to 500k.
Next create the ad. Your ad copy should start with a burning question. Clear benefits (3-4) to reader if they read your book. And end with a call to action to buy your book.
Non-Fiction ex.: Are you feeling stuck on the journey of life? ... Get clarity on what you need to do immediately after reading the book, etc ... Click below to own your copy today.
2. Book Giveaways:
The second strategy is to use book giveaways. But not just any generic giveaway, but a specific, targeted giveaway.
Ex. Giveaway 10 Print books in your genre.
Go to the Bestselling list of your genre on Amazon. You will see another list. Most wished for in that genre. Open that list and select the top ten books. These will be best selling books by famous authors that readers want to read.
Set up a giveaway using Rafflecopter or Kingsumo. Either use Facebook ads to target your audience or ask among your FB friends who read that genre to share the giveaway.
You will be able to grow an email list, share your giveaway progress, and announce the winner. The beauty of Rafflecopter and Kingsumo is that they incentivize sharing. In other words, if the readers who sign up to the giveaway share it with their friends, they get more chances to win your prize. This email list will also be helpful once you get a second book and email about it to them.
Hope this helped. If you have any follow up questions, please let me know.
I actually just answered this question with a video I recently posted on my YouTube channel Author Journey. I share how I made $7,000 in one month during a launch and how to utilize that for a preorder launch. You can check that video out at the link below. https://youtu.be/bJhd8DgzIHg