The Book of Invasions: A Celtic Symphony is the sixth album by the IrishCeltic rock band Horslips. It was a concept album based on an adaptation of Irish legends built into a complex story. It is named for the Lebor Gabála Érenn, a book of Irish mythology known as The Book of Invasions in English. Released in 1976, it is usually considered their best work. It was their only UK top-40 album, peaking at #39. "Trouble (With a Capital T)" and "The Power and the Glory" were released as singles.
The 30th anniversary of this album was celebrated at a small gathering in Dublin organised by Horslips fans and was attended by some band members.