Shopping Malls when I worked at shopping malls I would get someone trying to recruit me to join their company about once a month. So I would frequent your local mall and find people who go above and beyond in retail.
Look for Rapport building skills
Do they listen and help direct you to products
Do they ask open ended questions?

Often Retail Store managers were the best sales person for that store an other and they got promoted.

These people will have the soft skills of selling and then can be training in the hard skills and tactics of B2B Selling. If you have anymore questions or need any help I'm happy to get on a call.

So there's a lot of different ways you could do this, you could do this via cold e-mailing business you think would be a good fit with a sequence explaining the benefits of posting daily to Facebook and Twitter, The Potential ROI, and the benefit of hiring you to do it rather than managing it themselves or hiring another freelancer. You can also find plenty of people looking for this service on places like upwork, linkedin groups, and facebook groups. Lead with value and benefits and you'll win more clients.
If you want more help I'm happy to hop on a call.

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.