How Do Professional Book Marketing Services Work?

I’ve been looking into book marketing services in USA, but I’m still trying to understand how they actually work. Do they just promote books through ads, or do they offer a full strategy, including PR, social media, and Amazon optimization?

I’ve seen some services that help with book reviews, email campaigns, and influencer outreach, but I’m wondering—what’s actually worth investing in? Also, do these services work better for self-published authors, or can traditionally published authors benefit from them too?

For those who have used a book marketing service, what was your experience like? Did it help boost sales and visibility? Were there any specific strategies that worked best?

I’d love to hear from authors, marketers, and anyone who’s been through this process!