Published in 1939, it was the very first in the King Penguin series, which ended in 1959. King Penguins were the first hardcovers Penguin published, though only those after 1949 had dust jackets. They were also the first Penguins with colour printing. Talking of which, the plates in this one, from John Gould’s The Birds of Great Britain (1873), are fabulous:
Looks like I might have to start collecting the other 75 titles in the series. Or maybe I’ll just look out for no.11, Fishes of Britain’s Rivers and Lakes.