This article is from the Piano General Topics FAQ, by Isako Hoshino email@example.com with numerous contributions by others.
Paul Cooper "Perspectives in Music Theory", Harper & Row, 1973.
book on music theory
Benjamin, Horvit, and Nelson, "Techniques and Materials of
Tonal Music", Houghton Mifflin, 1975.
more book on music theory
John Clough and Joyce Conley, "Scales Intervals Keys Triads
Rhythm and Meter", Norton. ISBN 0-393-95189-8, ~$24
programmed text for introductory theory
Dorothy Priesing and Libbie Techlin, "Language of the Piano:
A Workbook in Theory and Keyboard Harmony", Carl Fischer
focuses on theathing theory in a way that is useful to
keyboard players. Covers theory and some keyboard