One for the Americans (and book geeks)

I'm redating and reposting this because I suddenly realized that most of the people who I need to see and comment on this were likely to have been off doing stuff over the holiday weekend. I really could do with some thoughts here people, it's for a charity I'm connected with who want to get the best return from a donation.

If you found an old and possibly rare book of historical interest to Americans which auction house would you take it to.