Roughing It by Mark Twain (1872) — Twain spent time in the California and Nevada mining camps in the 1860s, and his account of that period is still the sharpest, funniest, and most penetrating record of what Gold Rush country actually looked like. Grass Valley's Holbrooke Hotel saloon would have been familiar territory.
The World Rushed In by J.S. Holliday (1981) — A social history of the California Gold Rush built around the diary of a forty-niner from Connecticut. Better than any academic account at capturing what drove people here and what the experience felt like on the ground.
Gold Rush Country by the editors of Sunset Magazine — A practical and well-illustrated guide to the whole Gold Country region, useful for planning a longer exploration of Highway 49 that takes in Grass Valley, Nevada City, and the mining towns to the south.