Diamond's Edge. Poster of the 1988 movie and cover of the 1986 book, The Falcon's Malteser
VS3
The Falcon's Malteser
Anthony Horowitz, 1986
movie vs book
Stephen Bayly, 1988

Thirteen-year-old Nick and his slightly dense older brother Herbert run the Diamond Private Detective Agency above Camden Town Tube Station in north-central London. When a master criminal called The Falcon dies, they come into possession of his box of chocolate Maltesers, which contains the secret key to a fabulous cache of diamonds.