The Promised Land by Władysław Reymont (1899) — Reymont's Nobel Prize-winning novel about three young men — a Pole, a German, and a Jew — who build a textile fortune in Łódź while losing their souls in the process. It's a devastating portrait of industrial capitalism and ethnic tension set in the exact world that created the city's palaces and slums. Andrzej Wajda made a fine film of it in 1975. Essential reading before visiting.
The Painted Bird by Jerzy Kosiński (1965) — Not set in Łódź specifically, but this harrowing novel about a child's survival in wartime Poland captures something of the horror that engulfed this region during the Second World War. Kosiński grew up in the Łódź area and the book, however controversial, is rooted in a real landscape.
A World Apart by Gustav Herling-Grudziński (1951) — A memoir of Soviet imprisonment written by a Polish writer who survived both the Soviet gulag and the German occupation. Not about Łódź directly, but gives context for what the city's population experienced across two brutal occupations during the 1940s.