The publishing industry veterans I've worked with told me it was even more incredible during the height of the dotcom boom: book sales in the 100,000 copy range was not that rare.
Today I can only think of two truly technical book stores that still exist: The MIT Press Bookstore in Cambridge, MA and Ada Books in Seattle, WA. The latter, while a delightful store, has relegated the true technical book section to the backroom, which unfortunately doesn't seem to get refreshed too often (though, part of the beauty of this is it still has many of the weird old technical books that used to be everywhere).