I do love Oliver Sacks' work so I will look for that book, McGrath...

as for the "break a leg" custom, as a former acting student, I was always told the superstitious tardiitons of the theatre are to help prevent things from going wrong. By wishing someone to breaka leg, supposedly you helpo insure that all will go well! As for the phrase's origin, I don't know precisely but was once told it referred to an actual leg-breaking accident on stage...