Eclipse of the Crescent Moon by Géza Gárdonyi (1901) is the essential Eger book — a historical novel about the 1552 siege that remains one of the most popular books ever written in Hungarian. Gárdonyi lived in Eger for the last thirty years of his life and is buried in the castle. The novel was translated into English and is still in print; read it before arriving and the castle and its legends will mean considerably more.
The Invisible Man by Géza Gárdonyi (originally published in Hungarian as Láthatatlan ember, 1902) is less well known internationally but gives a picture of late antique life along the Danube and Tisza rivers that provides historical context for the whole region.