"Book of Days" is a hit song released in two versions, both composed and recorded by Irish musician Enya. The original version, included on the 1991 album Shepherd Moons, was sung in Irish Gaelic. The subsequent 1992 single version was bilingual, with new English lyrics; this version was recorded for Ron Howard's film Far and Away, and scenes from the film feature in the video. The bilingual English-Irish version (2:56) replaced the original pure Gaelic version (2:34) on subsequent pressings of Shepherd Moons from mid-1992 onwards, making the original recording relatively rare.

It was used as background music during the Medal Ceremonies at the 2009 IAAF World Championships in Athletics in Berlin.

It was also used as a temp track during the editing of the 1997 film Titanic, for the scene eventually accompanied by the musical cue called "Take Her To Sea, Mr. Murdoch" on the finished soundtrack by James Horner.

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