The name of the Saturnian moon Iapetus is spelled Japetus in the book. This is an alternative rendering of the name, which derives from the fact that "consonantal I" often stands for "J" in the Latin language (see modern spelling of Latin).
In his exhaustive book on the film, The Making of Kubrick's 2001, author Jerome Agel discusses the point that Iapetus is the most common rendering of the name, according to many sources, including the Oxford English Dictionary. He goes on to say that "Clarke, the perfectionist", spells it Japetus. Agel then cites the dictionary that defines jape as "to jest; to joke; to mock or make fun of". He then asks the reader, "Is Clarke trying to tell us something?"
Clarke himself directly addressed the spelling issue in chapter 19 of The Lost Worlds of 2001, explaining that he simply (and unconsciously) used the spelling he was familiar with from The Conquest of Space (1949) by Willy Ley and Chesley Bonestell, presuming that the "J" form is the German rendering of the Greek.