The Divan of Hafez. No book is more Shirazi than this. The 14th-century ghazals of Hafez are the soundtrack of the city, recited at his tomb, opened at random for fortunes in homes across Iran, quoted in everyday conversation. Dick Davis's English translation captures the music as well as any.
The Gulistan and Bustan by Saadi. The "Rose Garden" and "Orchard" of Saadi, the 13th-century moralist and traveller, were the literary education of every Persian-speaker for centuries. Edward Eastwick's older translation is freely available; W. M. Thackston's modern version is clearer.
Black Garden by Thomas de Waal. Not about Shiraz specifically, but the best introduction in English to the region's complicated 20th-century politics and the Iran–Caucasus borderlands.
Persepolis by Marjane Satrapi. The graphic memoir of a girl growing up in Iran during and after the Revolution. A reminder, when walking past Shah Cheragh, of what Iran has been through in the last fifty years.
The Pilgrim's Tale: A Year in the Life of Hafez by Faridoun Farrokh. Reads as a novel; gives a tangible sense of what 14th-century Shiraz might have felt like.