The Hitchhiker's Guide to the Galaxy (in-universe)

The Hitchhiker's Guide to the Galaxy was a so-called "trilogy" (actually a quintilogy) of books written by Douglas Adams, a Human from Earth, in the late 20th century.

The Doctor was aware of the book, and he once read a book - "Origins of the Universe" by Oolon Coluphid, a character in the books. However, as the Doctor noted, he "got it wrong on the first line". (DW: Destiny of the Daleks)

The Doctor once compared himself to Arthur Dent, the main character of the books, after saving the Earth from invasion at the hands of the Sycorax in a dressing gown, and after being awoken from his post-regenerative trauma by tea, the character's favourite drink. (DW: The Christmas Invasion)

Behind the Scenes

Douglas Adams was a writer of three Doctor Who television serials of the Fourth Doctor's era - The Pirate Planet, City of Death, and Shada - and was a script editor until the middle of the Seventh Doctor's era, so he occasionally placed references into the scripts he was editing.