From TARDIS Index File, the free Doctor Who reference.

This article is written from the Real World point of view.

The Adventures of K9 was a series of four illustrated paperback storybooks for children written by David Martin and published by Sparrow Books in 1980. The four books feature K-9, the character Martin co-created.

Specifically, the stories (according to the Discontinuity Guide website) feature K-9 Mark I and are supposed to take place sometime after the events of DW: The Invasion of Time when the Doctor left K-9 on Gallifrey.[1]

The books include prefaces that indicate that K-9 is working on behalf of the Time Lords (which opens the possibility that this could also be K-9 Mark II, depicted in many spin-off media as eventually returning to Gallifrey with Romana who becomes Lord President.

A total of four books were published:

1. K9 and the Time Trap
2. K9 and the Beasts of Vega
3. K9 and the Zeta Rescue
4. K9 and the Missing Planet

Although the general spelling used for the name K-9 is hyphenated, these books consistently use the unhyphenated form, K9. They also suggest that the Doctor's real name might be the Theta Sigma nickname heard in The Invasion of Time and DW: The Happiness Patrol.