Dream of the Red Chamber by Cao Xueqin (18th century) is China's greatest novel, set partly in Jiangnan and saturated with the culture, garden aesthetics, and poetry of the region that produced Hangzhou. Reading it gives the deepest possible context for the lake pavilions and classical gardens you'll wander through.
West Lake — a collection of classical Chinese poetry and prose about the lake, compiled across dynasties and available in various translations. The lake has inspired more Chinese writing than almost any other single place; the poems of Su Dongpo (who served as governor of Hangzhou twice and gave his name to the Sudi causeway and Dongpo pork) are the core of this tradition.
The Travels of Marco Polo retains its power in Hangzhou — this is "Quinsai," the city he called the finest in the world. Whether you believe his account or not, reading his description of the lake and its markets before you arrive sharpens the experience considerably.