City of Quartz by Mike Davis — Essential critical geography of Southern California, covering the politics, economics, and mythology of the region including the Inland Empire. Davis is polemical and sometimes overstated, but no book captures the darker currents of the greater Los Angeles basin as well.
Slouching Towards Bethlehem by Joan Didion — A collection of essays on California in the 1960s that still reads as a definitive portrait of the state's self-image and its fractures. Not specific to Riverside, but essential context for understanding the region's spirit.
California: A History by Kevin Starr — The definitive one-volume history of the state, tracing California from the Spanish missions through the twentieth century. Starr is authoritative without being dry, and the chapters on the agricultural and citrus booms illuminate exactly the history that Riverside embodies.
Hiking the Inland Empire — Various editions cover the mountain trails, desert edges, and riverside parks within an hour of Riverside, including routes in the San Bernardino Mountains and the foothills above the valley. Useful if you plan to combine a city visit with time outdoors.