CB01: The first sketch, testing the idea of measuring the resistance of conductive paint on the top edge to determine which spread the book is opened to. A conventional textblock is made from heavy-stock paper.
The top edge of the textblock is sanded and painted with BarePaint, a cheap graphite-based conductive paint.
A headband is stitched using conductive thread and fixed to the back in contact with the painted top edge.
The resistance is measured between the headband and a contact touching the first page for different opening points. It turns out that there is no consistency; the resistance values vary wildly between measurements. The values also vary during a measurement depending on pressure from above and on opening angle. The main sources of this variation are probably the resistance in the connection between contact and first page, and the resistances between individual pages.
The finished book, with simple programs for an Arduino microcontroller to measure the resistance of the book's top edge and for Processing to monitor the value and its running average. The book behaves mostly as a pressure sensor for the left hand, and it remains to be seen how it can be used.
CB04: Using photoresistors embedded in the inside cover, and holes in the pages covered with semi-transparent paper, it actually seems possible to build a book that would know which spread it is on.
The approach could scale to fairly thick books, but that would require a large number of holes in the pages, which would be rather defacing (unless they were made part of the typography!).