The Forging of a Rebel by Arturo Barea (1943) — Barea's three-part memoir covers his childhood in rural Castile, his years as a soldier in Morocco, and his work as a Republican press censor in Madrid during the Civil War. It's the definitive literary account of what it meant to be Spanish in the first half of the 20th century, and much of its texture comes from the Castilian landscape and provincial life that Cuenca represents.
South from Granada by Gerald Brenan (1957) — Brenan spent years in a remote Andalusian village and wrote what many consider the best English-language account of rural Spanish life. Though set further south, his observations about the isolation and self-sufficiency of inland Castilian communities translate directly to the world you find in Cuenca. Readable and sympathetic.
The Quest for El Cid by Richard Fletcher (1989) — The medieval Castilian warlord Rodrigo Díaz de Vivar — El Cid — operated across exactly the territory that includes Cuenca, which was a Moorish stronghold until the 12th century. Fletcher's biography is scholarly but genuinely readable, and it illuminates the world in which Cuenca's cathedral and castle were built. Understanding the Reconquista gives the old city an extra layer of meaning.
Homage to Catalonia by George Orwell (1938) — Strictly an Aragon and Barcelona book rather than Castile, but Orwell's account of the Civil War is still the best English entry point to the political catastrophe that shaped modern Spain. Reading it before visiting any Spanish city that lived through the 1930s — as Cuenca did — helps make sense of what you see and hear.