A good place to start is an article in the
American Society of Arms Collectors (ASAC)
Bulletin 14:18-23,
Indian Trade Guns, by Charles Hanson. There were many types of Indian trade guns, and Charles Hanson, of all people, would have known that, but this article is specifically about Northwest guns. Whoever transcribed and then digitized it got a little careless, but it is all there.
The definitive reference is
The Northwest Gun, a book by the same Charles Hanson. This book may give you an information overload, but I think it remains the best overall reference on this particular gun. Unfortunately, I think this book is out of print, but there may be a few still floating around out there, and there are also used copies available online.
Biblio.com has one in fine condition listed for $20, and if you are seriously interested, I would recommend that you buy it now.
There are also several articles in the Canadian Journal of Arms Collecting.
Joe Salter Antiques has just about all of them, for $5 per copy. Some that I would recommend are:
Vol. 2 No. 3 (August 1964), "Guns of the Northwest Company"
Vol. 11 No. 4 (Nov. 1973), "Guns of the American Indians"
Vol. 13 No. 3 (Aug 1975), "Trade Guns of the Hudson's Bay Company"
The books mentioned in the posts above by
@rich pierce and
@tenngun are excellent. In addition,
Trade Guns of the Hudson's Bay Company 1670-1970, by James Gooding, is a great resource, although I think it may also be out of print. It covers all sorts of guns and accoutrements sold by the HBC, but there is a tremendous amount of information about Northwest Guns in it.
If you cast a wider net, you'll find a lot more. These suggestions ought to get you started.
Best regards,
Notchy Bob