The Book of Disquiet by Fernando Pessoa — Pessoa lived in Lisbon, not Porto, but his fragmented, melancholic prose record of a city and a sensibility is the essential literary companion to understanding Portugal and the Portuguese interior. Read the prologue before walking the Ribeira.
The Gospel According to Jesus Christ by José Saramago — Saramago's most controversial novel reworks the Gospels with the cool revisionism he applied to much of Portuguese history. Any Saramago novel serves as preparation for Portugal; this one's irreverence captures the country's complex relationship with its Catholic heritage.
A Small Death in Lisbon by Robert Wilson — A historical thriller that moves between Porto and Lisbon, the Nazi gold trade through Portugal, and a present-day murder investigation. Gives good texture to the wartime period when Portugal's neutrality made it one of Europe's most important espionage theaters.
Port and the Douro by Richard Mayson — The standard English-language reference on port wine, the Douro Valley, and the history of the trade. Not light reading, but essential context before visiting the lodges in Gaia or the quintas upriver.
Portugal: A Companion History by José Hermano Saraiva — A readable overview of Portuguese history from its origins to the 20th century, covering the Age of Discoveries, the Inquisition, the Salazar period, and the 1974 revolution. Good preparation for understanding why Portugal's relationship with its past is complicated.