Key to North American birds containing a concise account of every species of living and fossil bird at present known from the continent north of the Mexican and United States boundary.

Coues continued to revise and expand this work, putting it through four editions during his lifetime. Digital facsimile of the 1872 edition from the Hathi Trust at this link. The Hathi Trust also makes available digital copies of the later editions, through the posthumous sixth edition (1927).