Here are a few books I have found helpful and interesting as a pianist. Many of these are available either through www.imslp.org or www.archive.org.
The Art of Piano Playing: A Scientific Approach by George Kochevitsky
This is a fascinating book that starts with the premise that piano technique begins in the brain. There are many helpful practicing techniques outlined that are not always intuitive but immensely helpful.
Piano Playing with Piano Questions Answered by Josef Hofmann
A book on piano playing by one of the best pianists of all time. An invaluable resource.
Basic Principles in Pianoforte Playing by Josef Lhévinne
Lhévinne, a great-grand teacher of mine, delves into issues of technique, musicianship, rhythm, tone production, and much more.
More to come…