His best known achievement is conceiving and in large part undertaking the six volume Anglo-Saxon Poetic Records edition (begun in 1931, and concluded by Krapp's collaborator Elliott Van Kirk Dobbie in 1953). Krapp is also noted for his books ''Modern English: Its Growth and Present Use'' (1909) in which he argued "that 'good English' was not determined by the conformity to grammatical laws, but by the common use of language", and ''The English Language in America'' (1925) described by Henry Blake Fuller as "detailing the adventures of an old language in a new country" and a book that "contravenes many of our favorite notions about ourselves and our speech". He wrote six children's books about subjects such as "the Civil War, the Great Lakes, the frontier, and country life".
In June 1924, Krapp wrote in ''The American Mercury'' several words in African American dialect were from common English usage that lingered in "Negro speech" while becoming archaic elsewhere. The article was noted in Monroe Work's ''Negro Yearbook 1925-1926'' (page 45) and elsewhere.