The Little Flowers of St. Francis (Fioretti di San Francesco). The classic 14th-century collection of anecdotes about Francis and his early companions — short, vivid, often very funny tales like the sermon to the birds and the taming of the wolf of Gubbio. The single best book to read before visiting.
The Life of St. Francis by St. Bonaventure. The official medieval biography, written by a successor as Minister General of the Franciscans. Drier than the Fioretti but the definitive theological framing of Francis's life.
Francis: A Life in Songs by Ann Wroe. A modern poetic biography that uses the medieval sources to create a portrait of Francis that is both rigorous and surprising. Excellent for understanding the man behind the legend.
Reluctant Saint: The Life of Francis of Assisi by Donald Spoto. A clear, well-researched popular biography that demystifies Francis without diminishing him. Useful for non-religious readers approaching the saint for the first time.
A Traveller in Italy by H. V. Morton. A 1964 classic of English travel writing about Italy; the Assisi chapter remains one of the most evocative descriptions of the town in any language.