Yes they are american, and really really good stuff. All except the White Mountains Trilogy which is sort of a kids science fiction (and decidely British), al beit with better developed characters, and not so allegorical.
I too read at least 30 picture books a week, and it would be crazy to list those, but suffice it to say there are some great reads in there.
Definately check out the Rootabega stories. Thay are awesome. Being a poet, Sandburg tells the stories in a rather poetic way, with lots of repetitions. And characters include talking brooms, corn fairies, Rootabega kings, and a kid that plays his "spanish spinish splishy guitars made special" with his mittens on, because of the cold bitter cold. A wacky world where the plot is dreamlike, and the characters even more so.