I was asked why would you want to write a book on Transylvania? What makes you want to do something so different in the first place?
Well, it almost makes one go and write a book on Dracula and Hunyadi (The White Knight) where you write a fiction book about them fighting or something... wait that has been done hasn't it?
The other problem is that they probably never knew each other, but it is fiction right?
Still, it would be sort of fun to write about Transylvania, and many have, but as many have pointed out, fact can be stranger than fiction. After all with centuries of history, you can see that Transylvania has a lot of material to work with. Ryalty and people and events all seem to have taken place with ties to Transylvania.
The problem is that there is limited original documents that many can access, at the same time they are there and you will need to look for them.... learning both a bit of Hungarian and Romanian is a bonus. then the documents are of huge value, but then they are always of value to anyone who is wanting to write about Transylvania.