There are a lot of words left to write about Transylvania, and I am sure that focusing on one city would be very interesting, and people I am sure would enjoy reading about it. This is a niche market, but the books on specific cities in Transylvania are few and far between. Brasov for one is a fasinating city with a wide cast of famous, and not so famous people.
Mostly, I believe that there is an interest in both the city of Brasov and the person behind the myth of Dracula- Gothic lovers will rejoice at seeing the Black Church- a church which owes its name to the fire of 1689 which nearly destroyed the town. In other words, given the difference in subject matter with my previous book, there is more than one market for Transylvania. It will take a lot of writing and research to do this with care.
There are hundreds of old places to see in this old fortress city, and much of Brasov is the way it was hundreds of years ago. The most famous landmark, Bran Castle is close to Brasov as well, which makes it a prime tourism city.
publish books on Transylvania, (both the culture and the history of the area) and many of them are written because it caters to an audience that most authors would enjoy. The price that the writer faces is that most people don't want to spend as much money to buy a book as the author needs them to to earn a living publishing on a topic such as this.
This topic is small, but with several hundred cities to choose from this may be a small place to start as a writer who is writing about Romania, and Transylvania.