The Good Rain by Timothy Egan (1990) is the best starting point for understanding the landscape of the Pacific Northwest — its rivers, forests, mountains, and the human stories layered over them. Egan grew up in the region and writes about it with both affection and clear eyes. The Okanogan country is part of the larger story he tells about what the Northwest is and what it has become.
A Hiking Guide to the Pasayten Wilderness — several editions have been published over the years covering this remote corner of Washington. Any current edition will give you trail descriptions, distances, and access information for the wilderness that defines the country west of Okanogan. Read it before you go; cell service is not available in the backcountry.
Mourning Dove: A Salishan Autobiography edited by Jay Miller (1990) is the memoir of Christine Quintasket (Mourning Dove), a member of the Colville Confederated Tribes who grew up in the Okanogan Valley at the turn of the 20th century. She was the first published Native American woman novelist. Her account of life in this valley — the land, the seasons, the collision between traditional ways and settler culture — is irreplaceable as a record of what this country was and what it cost to remake it.