My First Summer in the Sierra by John Muir (1911). Muir's account of shepherding sheep through the California high country in 1869, written with the rapturous attention to volcanic landscape and alpine terrain that defined his work. The Sierra Nevada and the southern Cascades he describes share the same geological origins as the country around Susanville — reading this before or after a visit to Lassen sharpens the eye considerably.
Lassen Volcanic National Park: A Complete Hiker's Guide by Mike White. The practical companion for anyone spending time in the park. White covers all the major trails including the Lassen Peak summit route and the Bumpass Hell boardwalk, with enough geological and natural history context to make the landscape legible rather than just scenic. Useful before the drive south from Susanville.
The Natural History of the California High Desert by various contributors, published by the California Native Plant Society. The Honey Lake Valley sits at the intersection of the Great Basin and the southern Cascades, a transitional zone with its own ecology. This book explains the plants, birds, and wildlife of the high desert basins in plain language, and gives a traveller the vocabulary to understand what they are looking at when driving the empty stretches of US-395.