There are a lot of books to there on Transylvania, or more specifically Romania, except the problem is that as a case writing friend said, a lot of slanted views.
Take fr example a book written by a publisher within Romania, usually as he pointed out, it has a Romanian point of view.
Same with any book where the publisher happens to be Hungarian, or German.
Each has their own "story" to tell and it is up to the reader to figure out the facts. Original sources are great but should also b viewed with caution as the are also slanted in one way or another. Part of what makes writing about this area, so interesting is that people learn from reading about it, but Transylvania will always be an interesting topic to talk about.