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.