Over the years, Humboldt repeatedly expressed intentions to write a book on American languages. Although he did not follow through with his plans, we can reconstruct to considerable detail what his book would have looked like by building on his “Essai sur les langues du nouveau Continent” and other Americanistic writings. Humboldt’s book would accordingly have included the following major sections: an introduction addressing the comparative-contrastive study of American languages; a unit on their sociohistorical embeddings; a part on historical comparisons (with attention to their phonologies); a section on grammar (with attention to its internal analysis); and a unit on linguistic typology. Another topic might have been language contact. Major samples would have come from Nahuatl, other languages of Mexico (Cora, Tarahumara, Huastec, Totonac, Otomí, and perhaps Yucatec), Quechua and other languages of South America (Araucano [Mapuche], Guaraní, and Muisca), plus Massachusett, Mahican, and Onondaga of northeastern North America.