The Lost Symbol, developed under the working title The Solomon Key, is a 2009 novel by American fiction writer Dan Brown. It is a thriller set in Washington, D.C.

Released on September 15, 2009, it is the third Brown novel to involve the character of Harvard University symbologist Robert Langdon, following 2000’s Angels & Demons and 2003’s The Da Vinci Code. It had a first printing of 6.5 million (5 million in North America, 1.5 million in the UK), the largest in Doubleday history.
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