Confucius, The Analects — The foundational text: Confucius's conversations with his disciples, recorded by the disciples after his death. Read the translations by D.C. Lau (Penguin) or Edward Slingerland (Hackett) before visiting Qufu; the physical site makes much more sense when you know the book. The Analects is shorter than you expect and considerably weirder and more interesting.
Simon Leys, The Analects of Confucius (1997) — The best introductory translation and commentary, from a scholar who genuinely loved the text. Leys's commentary is erudite and opinionated, and his introduction on the history of Confucian scholarship is one of the best short intellectual histories available in English.
Deborah Sommer, Chinese Religion: An Anthology — Contextualizes Confucianism within the broader Chinese religious landscape. Useful for understanding how the Confucian, Taoist, and Buddhist traditions interweave in a site like Qufu.
Ann Paludan, The Chinese Spirit Road — Covers the stone avenues, spirit paths, and funerary monuments of Chinese history, providing excellent context for the Kong Forest's grave sculptures and the Mausoleum of Shaohao's ritual landscape. The best visual guide to the kind of material you will encounter in Qufu.