This is a book of technical studies to help photographers master their photography.
The concept of an etude comes from music where specific techniques are studied in isolation. The idea is that when the technique is later encountered in a piece of music, musicians can more easily use that technique and return their focus to the interpretation and expression of the music.
The idea of an etude, a technical study, can easily be applied to photography. Any exercise designed to enhance a particular photographic skill is a “photo etude”. The point of etudes is to focus attention on one particular technique.