The Secret Adversary (Tommy and Tuppence Series)

Set in 1919 London and various other outlying locales throughout Britain, young couple Tommy Beresford and Prudence "Tuppence" Cowley, out of work and money, form the "The Young Adventurers, Ltd." planning to hiring themselves out as "adventurers … willing to do anything, go anywhere ~ no unreasonable offer refused." Overheard they are put to work.