I have to ask one question about the first Science of Discworld book.....
In chapter 20 (A giant leap for mankind) it states that in April 1969 'Neil Armstrong stepped down onto the surface of the moon, fluffed his lines and made history'. Well correct me if I'm wrong (I'm old enough to remember this), but surely he took that first step on 20 July 1969.
Ysabell x
+ [p. 152] "In April 1969, Neil Armstrong stepped down on top the surface of the Moon, [...]"
I am not sure if this error has been fixed in later printings of the book (I have been told that it is still present in the 2002 paperback edition), but it definitely needs to be: the first Moon landing was in July 1969.