I would imagine that the UK is responsible for the greater annual sales of English books rather than the USA - as you pointed out, Tintin has never really made it over there. I think the problem is the heavily-saturated comic strip market, but that's beside the point. Most UK bookshops stock at least some of the books, and you can generally find Tintin in almost every public library.
Anyway, in answer to your questions, I've searched for some statistics, but unfortunately have only been able to find them relating to the French books. This is a list of what I've come up with, though, some of it may be of interest :
Sales (per year) :
· In 1945, 65,000 French albums sold
· In 1955, 430,000 " "
. In 1965, 1.37 million " "
. In 1975, 2.2 million " "
. In 1980, 3.7 million " "
. In 1984 3.2 million (following Hergé's death the previous year
. During the 1980s, sales dropped to 2 million per year, but increased again to over 3 million in the early 1990s following the release of the Ellipse-Nelvana series of cartoons
· The Tintin books make up 17% of Casterman's total sales revenue at present, which for a series with no truly new books isn't bad at all. This is a step down from the 1980s, when it was closer to 50% of total sales.
· The re-edition of Tintin & Alph-Art
was the 8th best-selling comic strip album of 2004 in the Francophone world.
· In French, Tintin au Congo
is the overall best-selling album.
· 44% of French homes possess at least one album (making the second most popular comic after Astérix, statistically)
· Almost 30% of French homes have the full series
· Objectif Tintin
· La Galerie Gutenberg
I think that there's a sales chart or something like that in the new edition of Le Monde d'Hergé
by Benoît Peeters ; I'll get it out tomorrow and see what it has to say.
Hope that's of help !