This could be useful in determining patterns of use!
Dirty Books: Quantifying Patterns of Use in Medieval Manuscripts Using a Densitometer
by Kathryn M. Rudy
"Early users of medieval books of hours and prayer books left signs of their reading in the form of fingerprints in the margins. The darkness of their fingerprints correlates to the intensity of their use and handling. A densitometer -- a machine that measures the darkness of a reflecting surface -- can reveal which texts a reader favored. This article introduces a new technique, densitometry, to measure a reader's response to various texts in a prayer book." DOI 10.5092/jhna.2010.2.1.1
Nuns sometimes made little decorative ovals of paper to place beneath their thumbs as they read through their prayer books, etc., but I cannot remember if those objects had a specific name.