A self-proclaimed "low-brow" historian, his topics included Ethan Allen, the railroads, the timber industry, the Wobblies, and eccentrics of the Pacific Northwest.
An early proponent of conservationism, Holbrook believed that Oregon's growing population would damage the state's environment.
He wrote for The Oregonian[1] for over thirty years, was featured in The New Yorker,[2] and authored over three dozen books.
He also produced a number of satirical paintings under the pseudonym of "Mr. Otis," in a style he called "primitive modern."
These paintings are still shown occasionally at the Portland Art Museum [3][4] or can be found at the University of Washington Libraries, Special Collections.