Secret Seven on the Trail
Enid Blyton died in 1968 but remains one of the best-known and best-loved writers of children's stories.  She is consistently voted a children's favourite in author polls.  She has over 600 children's books to her credit, including the Famous Five series, the Secret Seven series, the Naughtiest Girl series (all Hodder) and the Malory Towers and St Clare's series (both available from Hodder on audio).