Roy J. Snell 
Secret Mark [EPUB ebook] 

The third in a series of books revolving around cousins Marian and Lucile by author Roy J. Snell, in 'The Secret Mark’ we are reunited with Lucile, now working part-time at the library at her Chicago University. While at the library one night Lucile witnesses the theft of a valuable book from the library’s collection. Together with her friend and roommate Florence, Lucile vows to track down the thief and solve the mystery, as danger and adventure inevitably ensue. A gripping tale from the popular American author. –