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

The Hitchhiker's Guide to the Galaxy was a play that Romana II once chose to attend over A Midsummer Night's Dream. She called it a comparatively "much more useful play, and much funnier too". (PROSE: A Midsummer's Nightmare)

Professor Chronotis seemingly once replaced The Worshipful and Ancient Law of Gallifrey in the Panopticon Archives with a book, "an Earth classic by one of the greatest writers in that planet's history". Chronotis then went on to say "... terribly funny, terribly thoughtful, wish I could remember the name of it, something about thumbing a lift, and there were towels in it ... oh yes, of course, it's called The Hitch—" before being interrupted. (PROSE: Shada)

In the TARDIS library, Chris Cwej found a twentieth-century paperback that used the phrase "Mostly Harmless". This struck him as a good summation of how the Seventh Doctor and Roz Forrester had thought of him early in their travels, before they got to know him. Cwej was unsure whether the book had been fiction or non-fiction. (PROSE: Damaged Goods)

Behind the scenes
These are allusions to the stage play and first novel of the Hitchhiker's Guide to the Galaxy multi-media series by Douglas Adams, which has multiple points of connection in- and out-of-universe with Doctor Who.