After the advent of Intelligent thrillers involving cryptographic codes in the recent times, more and more books are set in that formula. The Four rules

(1) Period – Renaissance (there has to be pig latin, French, Hebrew, Greek, every language that Europeans spoke during that time)
(2) Theme – Involves some secret society or the other, concealing the ‘Truth’ that would shake the foundations of everybody else’s belief.
(3) Codes -Ofcourse, anagrams, Caesar cipher, anything. (Occasional and irrelevant Trivia on the iambic pentametre or Belladona plant, or even worse, the height of Napolean Bonaparte, is prominent. But ofcourse, I read these books only for the fascinating trivia, so no complaints!)
(4) Male protagonist studying art history/literature (and must talk as if he’d met Da Vinci or Dante personally) in an Ivy League American University.

The Rule of Four doesn’t deviate from this formula even to the slightest extent. To sum it up, One Rule to Rule them all: Hyped books always sell.