Purportedly an ancient magical text, written by King Solomon of Jerusalem.
Also known as The Lesser Key of Solomon and The Book of Evil Spirits, or Goetia the Lemegeton is a tome of demonic conjuration. It details the powers of 72 different specific "Spirits of Goetia" along with spells of summoning, constraint, departure, and command. There are also over two hundred talisman diagrams and a Qabbalistic chart, as well.
There is a known out of print version of the Lemegeton translated into English in 1903 by the widely known Rosicrucian and Hermeticist, Arthur Edward Waite. It was published by de Laurence, Scott and Company in 1916. This is the oldest copy I can find any reference to, and Waite himself states in the introduction that this is the first time the data has been available to "students of Talismanic Magic", but whether this implies that he wrote the text himself, or that there were only a few copies surviving at the time is subject to question.
There is also a paperback translation by MacGregor Mathers which is still in print as of the date of this writing. Given Mathers' dubious history, and the fact that I have not compared this version to any older translations, so I have no idea how it might compare to the 1916 printing.
Not to be confused with The Greater Key of Solomon, or simply, The Key of Solomon which is the second of the Keys and the subject of another node.
The actual origin of the Lemegeton is unknown.