Three other books of poetry are listed by booksellers in lists I found, Kadenzen, Heimkunft- Neue Kadenzen and Die Hafenorgel (with drawings by him). The lists show that the first and last had several printings. There are also a couple of short works illustrated with his drawings that may be poetical.
He is best known in America for his Story of the Gulf Stream; also translated and published in France and Britain. History of a City (Hamburg) was issued in a four language edition.
His novels do not seem to have made it into English, nor could I find his books of poetry in translation.