Here is an excellent article by Kenneth Ames (for free no less) that gives you a broad description of the prehistory of the Northwest Coast cultural area since original occupation. The Northwest Coast has been occupied for at least 14,000 years and may well have been the gateway into the New World. What Ames' article isn't so great at is highlighting the variability among Northwest groups in the later periods.
The Northwest Coast, and specifically many cultures in British Columbia, has been studied extensively by anthropologists of all variety from the late 19th century on. We have Franz Boas and his students (like Kroeber, Sapir and Teit) to thank for that.
With regard to your specific question (How much is known...?), I would say quite a bit, but I am not going to try to summarize it when Ken's paper is available for free. I will provide some highlights from my perspective on the west coast.
The earliest sites until recently were 9-11 thousand years old. Recently, a 14,000 year old component was found on the coast of British Columbia.
Cultural complexity and evidence of social stratification begins about 4,500 years ago and is well in place in most of the Northern Region by 3,500 years ago. It isn't until about 2,500 years ago that evidence of cultural compexity is widespread in the Southern Northwest Coast. The abundance of fish and sea mammals are thought to make possible the development of coastal and riverine nucleated sedentary sites (towns.)
The key to the development of complex culture in the NWC was the superabundance of marine/riverine resources and technologies that developed to exploit and store them. Among those technologies, salmon dams and traps, deepwater fishing and sea-mammal hunting were paramount.
Ornate woodworking technologies including large extended family houses and their decoration, canoes and carving of house poles, totem poles and boxes are hallmarks of NWC art and begins to develop around 3,000 years ago. The ornate house poles and totem poles did not arise until the very late prehistoric period.
The NWC held some of the densest populations in western North America in prehistory.
Social stratification was evident by at least 2500-3000 years ago as evidenced by decorative items in grave lots such as labrets and ear spools.
Throughout the vast majority of the NW Coast, from Northern California to Southern Alaska, the primary political and economic unit was the extended household. The emphasis on acquired wealth was strong and the wealthy families were revered and envied.
Recommended further reading: Ames and Maschner 1999 Peoples of the Northwest Coast; their archaeology and Prehistory
Matson and Coupland 1995 The Prehistory of the Northwest Coast
Lee Lyman 1991 Prehistory of the Oregon Coast; the effects of excavation strategies and assemblage size on archaeological inquiry.
Edit: Formatting, grammar and links.
Just fyi, there is an /r/AskAnthropology, but there are several specialists in aboriginal - including Canadian First Nations - history here
There are volumes of writings that give us an insight to how the Indians lived pre European contact. But these writings come in small snippets written in the journals, reports and lletters written by the early fur traders who lived among them.
My interest is in the Canadian fur trade with a large focus on the history of British Columbia. Here are a few books that are available for sale today or can be read online.
The British Columbia Chronicles volumes 1 and 2 by George and Helen Akrigg. These are compilation of short articles and writings that give a reader a year by year insight of the happenings in BC, Washington and Oregon from the first contact up until 1885.
The history of the Northern interior of British Columbia was written by Father A.G. Morice. This book gives us a first person account of the interior Indians life directly from Kwah, a Carrier Indian Chief who lived before and during the times first contact was made.
Samuel Hearne wrote a journal during his trip to the Arctic ocean in the 1770's and makes many notes about the behaviours of his Indian guides and packers.
Daniel Harmon lived with the Indians at Fort St James starting in 1810, just five years after first contact was made with the interior Indians. He writes of a young Chief Kwah and tells of watching the funeral of a revered Chief that was incredibly barbaric.