Norman Thelwell left teaching to take up illustration full-time in 1956 and his first book, a collection of his cartoons, Angels on Horseback, was published in 1957. The first book produced from cover to cover rather than collected from drawings previously published was A Leg at Each Corner, published in 1961. This was serialised in the Sunday Express and led to the development of the strip cartoon characters Penelope and Kipper.