at the risk of sounding insensitive, (it wont be the first time TBH) surely talking books are aimed mainly for blind people, so why would gas books be audio? this reminds me of a time when i asked students why they thought an appliance data badge had info stamped through from the back so that it was raised ( i assume it is so that it is more durable) nobody answered and nobody so i said irt was a form of braille for the blind gas engineers so that they knew the important info for servicing, nobody commented at all and it was only hours later someone said they didnt understand how a blind gas engineer could tell if a spillage test passed or not, i was nearly bent double when i said "ah but when you are blind your sense of smell is hightened and he would smell along the front of the fire and would know if all the smoke went up the flue" the guy just said oh ok and got on with what he was doing