1920s Role Playing game from TSR. Set in the gangster-ridden, Depression era, Prohibition America, Gangbusters has players taking on the roles of Law Enforcement (from beat cops to FBI agents to Elliot Ness style Untouchables) or Private professions (Private Eyes or Reporters), or Crime (from Petty Crime to full scale gangsters running all the rackets).
The book has some introduction for new players, but it's mostly setting and you'd need some RPG experience to start right away. The rules have all the basics (character creation, combat, skills, car chases), but there is a lot of material to guide you to gaining Experience Points in your chosen career. The emphasis is on action and fun.
The 3rd edition 128pg softback book has a lot of dense text, packing in all you need to run and play the game. There's a full colour double-sided grid map of city locations. The back cover also folds out, with a set of colour chits you can cut out to use on the map (sedans, armored cars and people). The last 50 pages are all material for a detailed campaign setting: Lakefront City.