Diamond Brothers, The Falcon's Malteser (1986) - Anthoney Horowitz

When the vertically-challenged Johnny Naples entrusts Tim Diamond with a package worth over three million pounds, he's making a big mistake. Tim Diamond is the worst detective in the world. Next day, Johnny's dead, Tim feels the heat, and his smart younger brother, Nick, gets the package and every crook in town on his back!
When a dwarf comes into the office and leaves a package, Tim Diamond, the world's worst private-detective, is faced with his toughest case yet. The office is ransacked and the package is found to contain simply a box of Maltesers. Who was the dwarf … and why was he murdered shortly after his visit?

Anthony Horowitz - Book 1