The Time Traveler's Wife

The Twelfth Doctor kept a copy of The Time Traveler's Wife in one of the bookcases in his TARDIS. He hid a TARDIS key key within its pages. (TV: Dark Water)

Behind the scenes
The novel was published in the early 21st century and written by Audrey Niffenegger. Steven Moffat aknowledged it among his inspirations, especially for The Girl in the Fireplace.