Gelett Burgess 
The Master of Mysteries [EPUB ebook] 

STRO PUT The Great Cryptogram back upon his book-shelf among the other attempts to solve the immortal Shakespeare-Bacon controversy.
“Valeska, ” he said, turning to his pretty assistant, “it’s queer that there appears to be no other book containing a secret message except the Shakespeare folios, isn’t it! It seems to me that I have heard it said that Chatterton had a cipher in one of his books, though; that’s the only other one I know of. Strange more authors haven’t done it!”
“Why?” Valeska asked, looking up from her catalogue. “Why should a writer put anything in that can’t go plainly in the body of the book, or, at least, in an introduction?”
“For many reasons: He may be ashamed of the book, or have some other reason for not acknowledging its authorship. It may describe his friends too accurately. It may reveal important secrets. Even if his name does appear on the title page, I can imagine of a number of secret messages he might want to insert for the benefit of those able to understand it.”