From TARDIS Index File, the free Doctor Who reference.

Harry Potter and the Deathly Hallows, published in 2007, was the seventh in a series of novels by J.K. Rowling chronicling the adventures of a young wizard named Harry Potter. The Doctor discussed the book with Martha Jones and said the ending made him cry. Soon after, he and Martha borrowed a few magical words from the Potter series to help William Shakespeare save London from a Carrionite invasion. (DW: The Shakespeare Code)

Although promoted at the time as the final book of the Harry Potter series, eventually 11 books were published in the series. (EDA: The Gallifrey Chronicles)

Real world

• The title of the book is never given on screen in The Shakespeare Code as at the time the episode was written and filmed, it hadn't yet been announced publicly. The book was released several months after the first broadcast of the episode.
• The episode does not definitively describe Deathly Hallows as the final book - the dialogue simply refers to it as "the seventh book", maintaining continuity with the novel The Gallifrey Chronicles which describes the Doctor as having a complete set of 11 Potter novels, suggesting that in the Whoniverse either Rowling herself wrote more books (as she has not completely dismissed doing in the real world), or someone else did.

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Harry Potter and the Deathly Hallows