The material of the Masonic apron-lambskin-is a symbol of innocence, as the lamb has always been.
Color and material are important in its symbolism but Masonry admits the "symbol of the symbol"--as, for instance, an electric light in place of a candle. Hence a Mason has more than once been "properly clothed" when the lambskin aprons of the lodge were all in use and he came through the tiled door clad in a white handkerchief!
Downloadable books (free):
John Sebastian Marlowe Ward - The Master Masons HandbookAleister Crowley - The Mass Of The Phoenix
Aleister Crowley - Why Jesus Wept