Hello tph
The most interesting book I ever read piano technique is Otto Ortmann's "The Physiological Mechanics of Piano Technique" It has 395 pages. It is an in depth scientific analysis of what happens when playing. Ortmann connected pianist's arms, hands, wrists and fingers to all sorts of instruments in order to register the activity of the muscles during playing and he draws conclusions from that. It was an attempt to clarify what really happens when you play.
Very interesting IMO is "The Pianists Touch" by Pichier and Krause. Also a detailed analysis of how to use the playing mecanism. What makes it very special too is the analysis of not only what the arms and fingers do, (like in most books) but much attention is given to the role of the shoulder blades, spine, legs, feet + muscles in pianoplaying.
The most usefull material for me were not my teachers or books, but a set of piano instruction video's + booklet from the Taubman Institute. When I applied the principles shown on the video's my technique improved instantly and enormously. Very expensive those video's but worth the money.
Hope this will help you in your search for great books.
Cheers
Dan