I think "know your audience" applies when you are writing a book in a niche market. Not as many people will buy a book about Transylvania, but it is a subjet that interests people.
I have a few ideas regarding writing a book about Transylvania:
1) Do not write about Dracula, or anything Dracula. I am sure that most people have seen a lot of novels that authors have written that are set in Transylvania. These are often well written, and many have been published but I don't believe they give readers a good idea of the real Transylvania.
2) Writing about the people of Transylvania is a good start. There are plenty of smaller groups who came from Transylvania: Saxons, Jews, Hungarians, Romanians, and that is just the beginning of the list. You could also write about the castles or the economy of an Eastern European country such as Romania.
3) History books with a focus on Transylvania can also sell well. My own book is out on Kindle, but it is available at some bookstores and is selling equally well there. Of course, Kindle is a good start and many people are willing to buy a history book when it is priced well.
4) Photography books: These sell well too, and Transylvania is certainly a place where you can take some impressive photos.
Become a Follower of Things about Transylvania, and read In Search of The Lost Ones: The German Soldiers of Transylvania in the Second World War and Their Stories and on Kindle In Search of The Lost Ones: The German Soldiers of Transylvania in the Second World War and Their Stories you can also find it here and here