Supplementary Volume 15 of the New Liszt Edition contains both earlier versions of published works and also several works that remained as manuscript. The most extensive work in this volume is the earliest known draft of the Piano Concerto in A major (written in 1839 for solo piano). Not only does the draft give insight into the process of the work's genesis, but it also holds its own as a work for piano solo. Additionally, the volume contains several shorter works, such as a piano transcription of Robert Schumann's Lied Widmung, which shows how original Liszt's interpretations were, and also two early drafts of the Zweite Elegie dedicated to Lina Ramann (a former pupil of Liszt's and his first biographer). The latter have been published in facsimile.

Supplementary Volume 15 features a detailed preface in German, English and Hungarian, which reveals valuable details on each work. The cloth-bound version also has critical notes in English.