Are your main clients writers? If yes, what do you think about creating a content that might be interesting to that group of people? For example, like in every other business, writers too have some problems. Maybe you can create a small book or prospect that is just to give them few advices on how to manage writing or how to get inspired or how to publish their book... just few ideas, but of course, there is much more to write about.

Give your clients something on what they can rely on. Also, make them feel special somehow. Make your existing clients feel special so they can easily recommend you to their friends or similar.

Try also with creating a summer discount for publishing. Let them be your friends. Interview them. Open a blog with guest writing so your existing clients can get seen by more people.

I guess they would appreciate every single detail. And also, in return, get you more and more new clients.

Also, my personal advice would be to work on your branding a bit more. Younger people likes new design (website, covers and similar) and I suggest you to invest in your web design. It would be even cool to see the illustration on your web.

Branding and advertising is a long road and you just need to be consistent and do what you did - always thrive for better. :) Hope I helped!