The Five Find-Outers and Dog (not to be confused with The Famous Five

The Famous Five (series)

The Famous Five is the name of a series of children's novels written by British author Enid Blyton. The first book, Five on a Treasure Island, was published in 1942....

), also known as the Enid Blyton Mystery Series, is a series of children's mystery

Mystery fiction

Mystery fiction is a loosely-defined term.1.It is often used as a synonym for detective fiction or crime fiction— in other words a novel or short story in which a detective investigates and solves a crime mystery. Sometimes mystery books are nonfiction...

 books written by Enid Blyton

Enid Blyton

Enid Blyton was an English children's writer also known as Mary Pollock.Noted for numerous series of books based on recurring characters and designed for different age groups,her books have enjoyed huge success in many parts of the world, and have sold over 600 million copies.One of Blyton's most...

 and first published between 1943 and 1961. Set in the fictitious village of Peterswood (Bourne End), close to Marlow, Buckinghamshire

Marlow, Buckinghamshire

Marlow is a town and civil parish within Wycombe district in south Buckinghamshire, England...

, the children encounter a mystery almost every school holiday, always solving the puzzle before Mr Goon, the unpleasant village policeman, much to his annoyance.
The Five Find-outers (and Buster the Dog):

• Fatty (Frederick Algernon Trotteville) – the leader of the Five Find-Outers from the third mystery on, when he justifies his leadership by demonstrating how to use invisible ink
  Invisible ink
  Invisible ink, also known as security ink, is a substance used for writing, which is invisible either on application or soon thereafter, and which later on can be made visible by some means. Invisible ink is one form of steganography, and it has been used in espionage...
  
   and how to escape from a locked room.