His research interests lied mainly in textual criticism of Greek literature — poetry in particular — and on the theory of textual criticism. He also wrote on Greek paleography and on the transmission and the reception of the classics through the
Middle Ages and the Byzantine millennium. Unlike most of his colleagues, he produced a relatively small number of
critical editions: of a collection of Byzantine liturgical poetry; of
Apollonius Dyscolus' treatise "On pronouns"; and of
Romanos the Melodist's poems, with Greek scholar
C. A. Trypanis. He wrote extensively on
Nonnus of Panopolis, although most of his conjectures and observations he never published and only wrote on the margins of the editions by
Arthur Ludwich and
Rudolf Keydell he owned. He produced similar works on
Apollonius of Rhodes,
Athenaeus, and
Herodotus. Classical scholar
Willy Theiler once observed that Maas's printed contributions to classical philology must be multiplied by ten, because most of Maas's reflections and thoughts were not published by him and still are not. His most famous work is the "Textkritik" (1927), a concise theoretical handbook of textual criticism. It has been translated into many languages. He also wrote a handbook of "Greek metre" (1923) and the handwritten notes for his unpublished "Byzantinische Metrik" have recently been discovered at
Copenhagen. His articles were collected by
Wolfgang Buchwald in 1973. == Maas's law ==