With the hunt on to solve the codes of the cover of TLS, it’s probably worth remembering that Dan Brown previously seemed to be building a system of hinting at the next book.
The Digital Fortress clue, “We are watching you,” gave us a hint of what was to come in Deception Point, where Dan Brown introduced the surveillance abilities of the National Reconnaissance Office, etc. The clue in Deception Point, “the Da Vinci Code will surface,” made no particular sense at the time, but WOW! It sure did later!
The clues on the jacket of DVC said things like, “Is there no help for the widow’s son?” and “e pluribus unum” and pointed to the location of the Kryptos sculpture. All of these can be found in TLS. So whatever clues emerge from this cover probably should be construed as possible hints for what Robert Langdon will do next.
David Shugarts, Contributing Editor, Secrets of the Lost Symbol
Secrets of The Lost Symbol 
Dan Brown gives us an interesting clue hidden within the arches of the back cover of The Lost Symbol which reads, “As Above…so below.” This is a reference to one of the principle tenets of Hermetic philosophy. In fact, many old etchings of Hermes Trismegistus that survive today include a similar phrase or include a symbolic allusion to it within the artwork. Could the next book that Dan Brown is writing include a plot that involves modern-day mystics and alchemists, or perhaps a Rosicrucian-like cult?