Timothy Pauketat, Cahokia: Ancient America's Great City on the Mississippi (2009) — The clearest single account of what Cahokia was and why it matters. Pauketat, one of the leading archaeologists of the site, makes a compelling case that this was a genuine urban civilization — not a village that got large, but a planned city with cosmological ambitions. Essential reading before a visit.
Rinita Dalan et al., Envisioning Cahokia: A Landscape Perspective (2003) — More technical than Pauketat's popular account, but indispensable for understanding how the Mississippians transformed the floodplain landscape over centuries of construction. Best read alongside a site visit.
John Madson, Where the Sky Began: Land of the Tallgrass Prairie (1982) — Not about Cahokia specifically, but the best book for understanding the landscape these people lived in. The Illinois prairie shaped everything from diet to trade to cosmology. Madson writes about the grassland with the kind of specificity that makes the archaeology make sense.
Mark Rose, The Bottom of the Sky (2002) — A novel set partly in the Metro East area exploring the Mississippi River valley through time. Fiction can get at things archaeology cannot — the feel of the floodplain at dawn, the weight of the river in summer.