An extraordinary document, a signed letter from the professors of the Conservatoire Impérial de Musique endorsing the keyboard method of the important French organist and composer who played a central role in the development of the French symphonic organ style. 1 page, 33 x 21 cm.. Ca. 1855, describing the method in detail, and signed in full below by many leading composers and musicians of the period, including Giacomo Meyerbeer, Adolphe Adam, Ambroise Thomas, Delphine Alart, Esprit Auber, Michele Carafa, Fromental Halévy, Henri Herz, F. Le Couppey, Antoine Marmontel, L. Massart, Henri Reber etc. Usual folds, in very fine condition. Sold together with a letter on black-bordered mourning stationary from Mme. Lefébure-Wély to the French music publisher Heugel, dated March 1870, approximately 3 months after the death of Louis James.