Between the Woods and the Water by Patrick Leigh Fermor. The second volume of Fermor's account of his 1933-34 walk from Holland to Constantinople covers a long stay in Transylvania, including a passage through the Saxon villages around Brasov. The country he describes — feudal landowners, gypsy bands, isolated Saxon settlements — was on the eve of its disappearance, and his account is one of the most evocative pieces of writing about pre-war Eastern Europe.
Dracula by Bram Stoker. The novel barely visits Transylvania directly, but it is what made the region a tourist destination. Reading it before a visit to Bran Castle gives you the right kind of half-skeptical, half-credulous frame for the whole vampire industry around it.
Along the Enchanted Way by William Blacker. An English shepherd-historian's account of his years living in a Maramures and southern Transylvanian village in the 1990s and 2000s, watching a centuries-old peasant life dissolve as Romania joined the modern world. Tender and clear-eyed.
The Saxons of Transylvania by Konrad Gündisch (in translation). A serious history of the German communities that founded Brasov and shaped its old town. Useful background for anyone interested in why the place looks the way it does.