San Manuel Bueno, Mártir by Miguel de Unamuno (1931). Unamuno was rector of the University of Salamanca and the most influential Spanish intellectual of the early 20th century — his statue stands on the Plaza de Anaya in front of the university. This short novel, his masterpiece, deals with faith and doubt through the story of a village priest; Unamuno's Spain is present on every page. His confrontation with Franco's general Millán-Astray in the university's assembly hall in October 1936 — "Venceréis, pero no convenceréis" (You will win, but you will not convince) — is one of the most dramatic episodes in modern Spanish history.
As I Walked Out One Midsummer Morning by Laurie Lee (1969). Lee's classic account of walking through Spain in 1935, with a violin and almost no money, passes through Castile and gives a vivid picture of the country on the eve of the Civil War. Essential reading before any journey through this part of Spain.
The Rough Guide to Spain or DK Eyewitness Spain. Both give thorough coverage of Salamanca and Castile y León, with practical detail on opening hours, tickets, and the lesser-known sights. The DK volume has good architectural diagrams of the cathedrals and the university façade.
The Art of God: Plateresque Architecture in Spain (various authors). The plateresque style — named for its resemblance to silversmith's work — reached its highest expression in Salamanca. Background reading on the style transforms a visit to the university façade and the Convento de San Esteban from admiration of general ornament into something you can read and understand.