Call Richardson Grove at 707-247-3318, or write to Sinkyone Wilderness State Park, c/o Richardson Grove State Park, 1600 U.S. Highway 101 #8, Garberville, CA 95542.
The Sinkyone Wilderness State Park brochure contains valuable information on the park’s natural and cultural history as well as tips for planning your visit.
Another good source of information is Save the Redwoods League http://www.savetheredwoods.org/park/sinkyone-wilderness-state-park.
To go deeper, check out these books:
• Chase, Joseph Smeaton, California Coast Trails: a horseback ride from Mexico to Oregon, Houghton-Mifflin, 1913. Chapter 21 describes Usal Road—and the people who lived along it—more than a century ago.
• Evarts, John, and Marjorie Popper, editors, Coast Redwood: A Natural and Cultural History, Cachuma Press, 2001. In chapter 6, pages 150–151 describe the fight for Sally Bell Grove and the expansion of Sinkyone Wilderness State Park in the 1980s.
• Lorentzen, Bob and Richard Nichols, Hiking the California Coastal Trail, Volume One: Oregon to Monterey, Coastwalk, 2002.
• McKinney, John, Day Hiker’s Guide to California Parks, The Trailmaster, 2007.