A bookshelf was a shelf, or set of shelves, containing books. Most libraries contained them, and they were a significant feature in most offices and homes on Earth.
The Seventh Doctor's yellow control room had a bookshelf located under one of the arches. (COMIC: The Armageddon Gambit) The Twelfth Doctor's control room contained a large bookshelf on the walkway. (TV: Deep Breath)
The TARDIS library was full of bookshelves, which Clara Oswald hid behind when her Time zombie entered the room. (TV: Journey to the Centre of the TARDIS)
Iris Wildthyme's double decker bus had a bookshelf containing her diares on the top floor. (PROSE: Enter Wildthyme)