The only place they registered at all was on a small black device called a Sub-Etha Sens-O-Matic which winked away quietly to itself. It nestled in the darkness inside a leather satchel which Ford Prefect wore habitually round his neck. The contents of Ford Prefect's satchel were quite interesting in fact and would have made any Earth physicist's eyes pop out of his head, which is why he always concealed them by keeping a couple of dog-eared scripts for plays he pretended he was auditioning for stuffed in the top. Besides the Sub-Etha Sens-O-Matic and the scripts he had an Electronic Thumb — a short squat black rod, smooth and matt with a couple of flat switches and dials at one end; he also had a device which looked rather like a largish electronic calculator. This had about a hundred tiny flat press buttons and a screen about four inches square on which any one of a million "pages" could be summoned at a moment's notice. It looked insanely complicated, and this was one of the reasons why the snug plastic cover it fitted into had the words Don't Panic printed on it in large friendly letters. The other reason was that this device was in fact that most remarkable of all books ever to come out of the great publishing corporations of Ursa Minor — The Hitch Hiker's Guide to the Galaxy. The reason why it was published in the form of a micro sub meson electronic component is that if it were printed in normal book form, an interstellar hitch hiker would require several inconveniently large buildings to carry it around in.
Douglas Adams, The Hitchhiker's Guide to the Galaxy, Chapter 3