Of course a PMP certification is useful in the publishing sector. You are working in project management after all and this is a degree in project management.
The fact that the PMP is not very well known in the publishing sector can even play to your advantage (you'll be one of the very few project managers in that sectors who have this certification).
So my advice to you is to go ahead with the PMP certification, you will not regret it and it'll be worth your time and your effort!