Five Have a Mystery to Solve (Famous Five)
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.