The Da Vinci Code was the novel that propelled Dan Brown into the major league of fiction authors in 2003.
What makes the Da Vinci Code such a great read is that Dan Brown ensures that everything is so believable. Events and ideas within it seem very plausible despite seeming incredible.
The main character in the book (and the inevitable film that followed) is one Professor Robert Langden normally found teaching students at Harvard but called to Paris at the behest of the Louvre Art Gallery (and the Police). They are anxious to use his expertise to solve a murder committed on their premises.
Langdon recruits Sophie Neveu to assist him as seeks an answer to the reason behind the killing via the works of Leonardo Da Vinci. Only the information revealed by the code puts them in mortal danger as they move around Europe.