The Immoralist by André Gide (1902) — Gide wrote parts of this novel while recovering from illness in Biskra, and the city's sensuous warmth, date groves, and the freedom he felt at the edge of the desert permeate the book. The narrator's awakening is inseparable from North Africa; the Biskra sections are some of the most atmospheric pages written about the Algerian south.
The Sheltering Sky by Paul Bowles (1949) — Not set in Biskra specifically, but this novel captures the existential pull of the Algerian and Moroccan Sahara better than anything else in English. Bowles understood the desert as a force that strips people down to their essential selves. Essential reading before any trip to the Saharan fringe.
Isabelle Eberhardt: A Biography by Cecily Mackworth (1951) — Eberhardt was the Swiss-Russian writer who disguised herself as an Arab man and wandered the Algerian Sahara in the late 19th century, eventually drowning in a flash flood at Ain Sefra. Her life and the world she moved through — French colonialism, desert mysticism, Sufi brotherhoods — illuminate the Algeria that Biskra was part of.