Hal Meredith 
The Ancient Monk [EPUB ebook] 

When a compromising letter falls into the hands of an unscrupulous Army captain, a viscount finds himself the victim of blackmail. For years he pays to keep the letter hidden, but when the captain dies, the only clue to finding it is an engraved ring. The viscount turns to master detective Sexton Blake, who must use his wit and literary knowledge to decipher the cryptic clues and locate the scandalous letter before it is discovered and the viscount is ruined.