What is the expected publication date for Dan Brown's newest book?

As of 2015, the expected publication date of Dan Brown's next novel has not been announced. The author had mentioned that it usually takes several years for him to write a book.

Dan Brown admitted that his next book is to continue the Robert Langdon series. The Robert Langdon series began in 2000 with "Angels & Demons." This was followed by "The Da Vinci Code" in 2003. "The Lost Symbol" and "Inferno," published in 2009 and 2013, respectively, are the last two books of the Robert Langdon series to date.

Dan Brown claims to have plans for at least 12 more novels. It is unknown if all of them are to be part of the Robert Langdon series.