"The Big Four" by Agatha Christie is a detective novel published in 1927. When a dying stranger staggers into Hercule Poirot's bedroom and whispers about an international crime cartel, the famous detective and his friend Hastings find themselves pursuing a shadowy organization led by four powerful masterminds. Their investigation takes them across countries as they uncover kidnappings, mysterious murders, and a deadly new weapon, all orchestrated by criminals who seem always one step ahead.
Wikipedia page about this book: en.wikipedia.org/wiki/The_Big_Four_(novel))