Apel, Willi. 1986. Medieval Music: Collected Articles and Reviews.  Stuttgart: Steiner Verlag.

Apel, Willi. 1978. The French, Italian and Latin Poems in 14th-Century Music. Journal of the Plainsong & Mediæval Music Society, 1: 39-56.

Apel, Willi. 1972. The History of Keyboard Music to 1700.  Bloomington, Indiana.

Apel, Willi (ed.) 1970-72. French Secular Compositions of the Fourteenth Century. Corpus mensurabilis musicae.  Neuhausen nr. Stuttgart: American Institute of Musicology.

Reaney, Gilbert. 1968. Notes on the Harmonic Technique of Guillaume de Machaut. Essays in Musicology: A Birthday Offering for Willi Apel, edited by Hans Tischler, 63-68. Bloomington: Indiana University School of Music.

Apel, Willi, and Archibald Thompson Davison (eds.) 1966. Historical Anthology of Music. Vol. I. Oriental, Medieval and Renaissance Music.  Cambridge, Massachusetts.

Apel, Willi (ed.) 1963. Keyboard Music of the Fourteenth & Fifteenth Centuries. Corpus of Early Keyboard Music.  American Institute of Musicology.

Apel, Willi. 1959. Remarks about the Isorhythmic Motet. Les Colloques de Wégimont II—1955, L'Ars nova: Recueil d'études sur la musique du XIVe siècle, 139-44(discussion144-48). Paris: Les Belles Lettres.

Apel, Willi. 1958. Gregorian Chant.  Bloomington and London.

Apel, Willi. 1957. Imitation in the Thirteenth and Fourteenth Centuries. Essays on Music in Honor of Archibald Thompson Davison by his Associates, 25-38. Cambridge, Mass.: Harvard University Department of Music.

Apel, Willi. 1956. The Earliest Polyphonic Composition and its Theoretical Background. Revue Belge de Musicologie, 10: .

Apel, Willi. 1955. French Music of the Fourteenth Century'. Journal of the American Musicological Society, 8: 70-71.

Apel, Willi (ed.) 1950. French Secular Music of the Late Fourteenth Century.  Cambridge, Massachusetts: The Medieval Academy of America.

Apel, Willi. 1942. (4th ed., 1949). The Notation of Polyphonic Music 900-1600.  Cambridge, Massachusetts.

Apel, Willi. 1938. The Partial Signatures in the Sources up to 1450. Acta musicologica, 10: 1-13.