The book I was referring to is called 'Aircraft Icing' by Terry T. Lankford.
To quote the relevant part...
'The icing envelope for cumuliform clouds is somewhat more narrow than for stratiform clouds. The minimum altitude is 4000ft, and maximum altitude is 24,000 ft. Icing is most frequently encountered (about 40 percent of the time) at ambient temperatures of 0 to -10 degrees C. At lower temperatures, the frequency of icing in clouds is much lower, about 6 percent at -30 degrees C.'
Associated with this comment is a graph showing the icing envelope which also precludes icing below 4000 ft at any temperature....
Aside from this bizarre statement, seems not a bad book.