The Companions of Doctor Who was an imprint of Target Books which published novels about the Doctor's companions after they had stopped travelling with him. It was notable for being the first series of books to include wholly original novels set in the Doctor Who universe.
Prior to the advent of the imprint, novel-length prose set in the Whoniverse was strictly reserved for novelisations of televised serials. By 1986, however, BBC Enterprises was willing to let Target tell a few of its own stories, although not with the Doctor as the central character.
The first release in the series was Turlough and the Earthlink Dilemma, focusing on Vislor Turlough after he left the Doctor. It stands as the longest novel published in the standard Target Books format which was in use from 1973-1991, though a few releases in the later Virgin Missing Adventures format were longer.
This book was followed by the second release of 1986, Harry Sullivan's War. As the title suggests, it focused on the military exploits of Harry Sullivan. It is notable for being one of only two instances in which an actor playing a regular character on Doctor Who was able to write an original, published work featuring that character. Author Ian Marter is joined only by Colin Baker in the distinction.
The third and final release broke the format of the imprint somewhat by being a novelisation of A Girl's Best Friend, the pilot episode of the aborted K9 and Company spin-off series. Unlike its predecessors, however, it was published in 1987.
Further books were planned, but not published. Thus, the imprint died after only about a year's operation.