Helmann, J. N. (1969). The consciously controlled piano tone: most natural approach to the problem of artistic piano playing. 2d ed., enl. and rev. n.p..
Helmann, Jacob N. The Consciously Controlled Piano Tone: Most Natural Approach to the Problem of Artistic Piano Playing. 2d ed., enl. and rev. n.p., 1969.
Warning: These citations may not always be complete (especially for serials).