I'm currently working with a publisher on a book that provides an intro C# and XML. The way that I have gauged what to put in is to think about what info I needed when I first started. That got me only so far so I took to the internet and started searching what kind of questions people are asking about handling XML. From there I was able to add topics/information that would solve problems of real people so that others who may have the same problem have a resource to go to.
I cant give you any pointers on what topics to cover. But there is certainly market. I know that data engineering and finding relevant resources is one of the challenges faced by many startups today