Is the Best Social Media Marketing Agency Worth It

I’m looking to promote my book online and keep seeing recommendations for the best social media marketing agency, but I’m not sure if hiring one is worth it. Can an agency really increase engagement and drive sales, or is it something authors can handle themselves? How do you identify a reliable agency that understands book marketing specifically? I’d love to hear from authors or marketers who have experience working with a top social media marketing agency what worked and what didn’t?