The Sun Also Rises by Ernest Hemingway (1926). The novel that made Pamplona famous. Jake Barnes and his friends travel from Paris to watch the San Fermín bullfights in the 1920s. Hemingway's flat, declarative prose and the novel's mood of exhausted pleasure still capture something real about the city and its festival. Read it before you go.
Death in the Afternoon by Ernest Hemingway (1932). Not fiction but an extended essay on bullfighting, written while Hemingway was still attending San Fermín regularly. The first hundred pages are as good a description of what actually happens in the bullring as anything in English. Useful if the bullfights are part of why you are going.
The Pilgrimage Road to Santiago: The Complete Cultural Handbook by David Gitlitz and Linda Kay Davidson. An exhaustive guide to the Camino Francés and its monuments, churches, and history. Not light reading, but if you are walking the route or simply want to understand what the medieval pilgrimage meant, this is the reference.
Navarra: Unique Autonomy by various authors. A collection of essays on Navarrese history, culture, and politics — covering the old kingdom, the relationship with the Basque Country, the fueros (the historic rights Navarre has always insisted on), and the modern autonomous community. Dry in places but genuinely informative about a region with an unusual political and cultural identity.