Does my UBC have to be about publishing a book?
The physical book you use as a calling card does not have to be about publishing or the types of services you can offer; it just needs to be something relevant to the person that you are talking to. Having a book that describes what you offer is a good way to introduce yourself as there will be a 100% alignment between what you are offering and what you are handing somebody so that they will remember you.
However, if you are going to be approaching a specific niche, then writing a book related to that niche works pretty well and can get you their attention. If you can solve an actual problem that they are having, they'll know like and trust you that much more when it comes time to offer your services.
If you are already a professional in another area, even if it isn't the same niche as the prospects you are approaching, you could also use a book related to your expertise in that area to demonstrate the type of quality and what you can do for that person.
If you are giving somebody a book that doesn't explicitly state the book production services you are offering, then get some sticky notes or bookmarks and have them say something along the lines of, “Like this book? Let me help you promote your business with your own book!” That way if they look at the book a week or a year later they'll be reminded why they have it and will get in touch with you.
The important thing about your ultimate business card is that you can show off the type of work that you do. It should be high quality, and should make people want to do business with you. The content of the book will work better for getting you new clients if it's relevant, so don't use a children's book you've published unless you are offering children's book services and that's the market you are going into.
A professional book about how to publish or related to the reasons to publish their own ultimate business card can serve as a great piece of sales material, but anything that shows somebody what they could have if they work with you even if it doesn't describe that process in the book will work well, especially for in-person networking.