Agatha Christie 
The Secret Adversary (Diversion Classics) [EPUB ebook] 

Featuring an appendix of discussion questions, the Diversion Classics edition is ideal for use in book groups and classrooms.
After the First World War, friends Tommy and Tuppence go into business as The Young Adventurers, ready to investigate mysterious occurrences. Their new venture leads them on a search for the missing cousin of an American millionaire and draws them into a plot fraught with danger. Christie’s sharp writing and command of mystery make this detective novel a must-read.