Book excerpt: Mountain biking was invented in Marin County, and this authoritative guide describes the area's 40 best runs, from well-known rides on Mount Tamalpais and Pine Mountain to off-the-beaten-track adventures. The authors describe each ride in detail, rate them for difficulty on a one-to-five scale, and illustrate each route with a high-quality relief map and an elevation profile. Seven appendices pinpoint top rides, provide tips for beginners, and much more.