There are numerous unofficial stories - fan fiction - in circulation. As far as officially sanctioned (BBC-licensed or approved) stories, the only one I'm aware of is "Birth of a Renegade," a short story by Eric Saward (script editor for Doctor Who) that was published in the Radio Times Doctor Who 20th Anniversary Special in November 1983. It has never been reprinted officially. In the story, the Master discovers that the Doctor (Fifth Doctor) has had some memories of his reasons for leaving Gallifrey wiped. This is the same story that suggested Susan wasn't the Doctor's biological granddaughter. Its canonicity, like all spin-off fiction (even licensed by the BBC), with relation to the TV series in unclear. "the Doctor's Wife" provides an alternate origin: the TARDIS and the Doctor both wanted to leave Gallifrey and explore, so she (the TARDIS) left herself unlocked so the Doctor could more easily steal her.