The specific mechanics of "mutating" the dominant to the tonic to maintain the fugue's tonal center.
Because the work is in the in many regions (including France), several reputable digital archives host high-quality scans: Traité de la fugue (Gédalge, André) - IMSLP
His treatise is prized because it focuses on "pure music," avoiding the influence of Impressionism to focus on the rigorous traditions of Saint-Saëns and Bach. The work was originally intended as a multi-volume set, though only the first part, dedicated to the academic fugue, was completed.