The Famous Five Series by Enid Blyton
Although Blyton wrote many 'series' stories (The Mystery, Secret Seven, The Adventure, Barney, St. Clares, Malory Towers) it is the Famous Five series which appears to be the most popular.
The first book appeared in September 1942 (dated August 1942) and was so popular that it was reprinted almost immediately in September and October of the same year. The book was illustrated by Eileen Soper, who was to illustrate all 21 titles, and whose drawings were so 'just right'.
Her original black and white line drawings, together with the coloured jackets fit perfectly with the stories. In the latest editions these original drawings have been used in colour.