John Buchan 
The Power-House [EPUB ebook] 

When his friend Charles Pitt-Heron vanishes mysteriously, Sir Edward Leithen is at first only mildly concerned. But a series of strange events that follow Pitt-Heron’s disappearance convinces Leithen that he is dealing with a sinister secret society. Their codename is ‘The Power-House’. The authorities are unable to act without evidence. As he gets deeper involved with the underworld, Leithen finds himself facing the enemy alone and in terrible danger.