In Search of Dracula by Radu Florescu and Raymond T. McNally (1972) — the book that started serious scholarly interest in separating the historical Vlad III from the Dracula of fiction. Florescu and McNally trace Vlad's life, his imprisonment at Corvin Castle, and his campaigns in Wallachia. Readable popular history rather than academic scholarship, and essential context for a visit to Hunedoara.
The Historian by Elizabeth Kostova (2005) — a novel in which a young American woman pursues the trail of Vlad the Impaler across Eastern Europe, including Transylvania. It is atmospheric, paced like a Victorian thriller, and does a better job than most fiction of capturing the particular quality of Transylvanian landscapes and medieval architecture. Not a guide to Hunedoara, but the sort of book that makes the castle feel larger.
John Hunyadi: Hungary in Late Medieval Europe by Pál Engel, translated by Tamás Pálosfalvi — a scholarly but accessible account of the man who built Corvin Castle into its current form. Hunyadi's career — regent of Hungary, crusader, defender of Belgrade against the Ottomans — is as dramatic as the castle he left behind. This is the most thorough account of his life in English.
Transylvania: Its Products and Its People by Charles Boner (1865) — an account of Transylvania by a Victorian travel writer who covered the region with care and genuine curiosity. The industrial transformation Boner observed in embryonic form has now played out completely in Hunedoara, making his descriptions of the region's older character all the more striking by contrast. Available free through Project Gutenberg.