None of J.K. Rowling’s novels follows Robert Langdon.
Robert Langdon is the fictional Harvard professor and symbologist created by American author Dan Brown. He is the protagonist of novels including Angels & Demons, The Da Vinci Code, The Lost Symbol, Inferno, and Origin. The character is therefore associated with Brown, not Rowling.
Rowling’s major published fiction includes the Harry Potter series, The Casual Vacancy, the children’s book The Ickabog, and the Cormoran Strike crime novels written under the name Robert Galbraith. The similar association between famous authors and recurring literary protagonists can make this a useful distinction, but there is no Rowling novel centered on Langdon. The answer is “None,” rather than a title from Rowling’s bibliography.