Fiction and literature
The Book of Disquiet by Fernando Pessoa is the defining work of Portuguese literary culture — a fragmentary, melancholy meditation on Lisbon and on being alive. Not a travel read in any conventional sense, but no book captures the city's atmosphere better.
Baltasar and Blimunda by José Saramago is a historical novel set in 18th-century Portugal, by the country's only Nobel laureate. His The History of the Siege of Lisbon is a more immediate Lisbon read.
Night Train to Lisbon by Pascal Mercier (Peter Bieri) follows a Swiss professor who impulsively boards a train to Lisbon. Atmospheric, if a bit slow — but it makes Lisbon feel worth running toward.
History
A Small Death in Lisbon by Robert Wilson is a thriller that weaves between the Salazar era and modern Lisbon. Good on the long shadow of the dictatorship.
The Last Crusade: The Epic Voyages of Vasco da Gama by Nigel Cliff puts the Age of Discovery in sharp relief, essential context for understanding Portugal's outsized place in world history.
Food and culture
Lisbon: Recipes from the Heart of Portugal by Rebecca Seal gives a solid grounding in Portuguese cuisine. For a more encyclopedic reference, My Portugal by George Mendes (a Portuguese-American chef) covers the food with depth.
Travel
The Rough Guide to Portugal and Lonely Planet Portugal are both kept reasonably up to date and worth having in print if you're doing extended travel. Rick Steves' Portugal is strong on Lisbon and the main tourist corridor and better-suited to first-timers.