Over the past few decades, Doctor Who has had a vast array of non-fiction and otherwise "factual" books released. These fall into two broad categories:
"Real world" factual books — non-fiction in the literal sense, these include histories of the programme, behind-the-scenes production information, media studies books, etc. etc.
"Fictional universe" factual books — i.e. books set inside the Doctor Who universe but which act as reference books for those fictional worlds — these would include books that outline the history of the Time Lords, the Daleks, etc.