Doctor Who Discovers

Doctor Who Discovers was a series of educational children's books written by the Fourth Doctor during his time working with UNIT. By one account, the Doctor was later contacted in his fifth incarnation to finish the series.

The intended title for the series was The Doctor, Who Discovers... but the publishing company misprinted the title, resulting in the author being mistakenly credited as "Doctor Who". (AUDIO: The Kingmaker)

Contents
The Fourth Doctor completed five books in the series: Doctor Who Discovers Prehistoric Monsters, Doctor Who Discovers Space, Doctor Who Discovers Strange and Mysterious Creatures and Doctor Who Discovers the Conquerors. He began work on the sixth, Doctor Who Discovers Historical Mysteries, but left it unfinished until a robot sent by the publishers from the 64th century invaded the TARDIS to forcefully remind the Fifth Doctor of his contractual obligation; the Doctor therefore resumed investigating the mystery of who killed the Princes in the Tower during the reign of King Richard III of England.

In the course of the adventure William Shakespeare died in the century before his birth, which cause the robot's deadline circuit to malfunction; the robot then exploded. Unable to write about the truth of what happened to Richard and the princes in order to preserve the course of history, the Doctor suggested the possibility of investigating the historical mystery of Jack the Ripper instead. (AUDIO: The Kingmaker)

However, according to another source, a sixth novel did exist written by the Fourth Doctor, titled Doctor Who Discovers Pirates. (PROSE: Doctor Who Discovers Pirates)

Behind the scenes

 * Doctor Who Discovers is a reference to a real world book series published by Target Books in the 1970s. As in-universe, they were a series of educational children's books narrated by the Fourth Doctor, of which only five were published. A sixth, Doctor Who Discovers Pirates, was written but shelved as part of the cancellation of the series due to poor sales.
 * The robot lists four books, which correspond to all bar Doctor Who Discovers: Early Man, but stops before it can reveal the fifth.
 * The Doctor's offhand comment that "it was either that or write a novelty cookbook" refers to The Doctor Who Cookbook.