Which Book Marketing Services Actually Help Authors Reach Real Readers?

I’ve been exploring different book marketing services, but it’s difficult to figure out which ones genuinely make a difference. What should authors look for targeted ads, social media campaigns, email outreach, or something else? Where can writers find trustworthy feedback about these services before investing money? And why do some marketing strategies work better for certain genres than others? I’m curious how other authors decide which marketing approach is truly worth it.