Harry Potter

Harry Potter was a series of very successful novels by J.K. Rowling that were published in the 1990s and 2000s and adapted into a successful series of films. The novels followed the life of a young wizard from childhood to young adulthood as he learned about his magical powers and dealt with the mechanations of the evil wizard who killed his family.

The Doctor had read the Harry Potter series of novels, and cried upon reading the ending of the seventh and final book, Harry Potter and the Deathly Hallows. Martha Jones also knew the books and suggested Potter's magic word "expelliarmus" to William Shakespeare to help defeat the carrionites (DW: The Shakespeare Code).

Behind the scenes
Before being cast at The Doctor, David Tennant played Barty Crouch Jr. in the 2005 film adaptation of Harry Potter and the Goblet of Fire.

The Shakespeare Code, in which the Doctor references reading the final book, was broadcast before the book was published, during a period of intense anticipation for its release.