The Book of the Law, written by Aleister Crowley, was the book that established the religion of Thelema. The full title of the book is: Liber AL vel Legis, sub figura CCXX. It was written by Aleister Crowley in Cairo, Egypt, in 1904. The book contains three chapters, each of which was written down in one hour. Crowley claims that the book was dictated to him by an entity named Aiwass, his Holy Guardian Angel.
He wrote the Book of the Law on April 8, 9, and 10, between the hours of noon and 1:00 p.m..
In "The Equinox of the Gods" he describes his encounter with Aiwass as follows:

"The Voice of Aiwass came apparently from over my left shoulder,
from the furthest corner of the room. It seemed to echo itself
in my physical heart in a very strange manner, hard to describe.
I have noticed a similar phenomenon when I have been waiting for
a message fraught with great hope or dread. The voice was
passionately poured, as if Aiwass were alert about the time limit.
I wrote 65 pages of this present essay (at about my usual
rate of composition) in about I0 1/2 hours as against the 3 hours
of the 65 pages of the Book of the Law. I was pushed hard to
keep the pace ; the MS. shows it clearly enough.
The voice was of deep timbre, musical and expressive, its tones
solemn, volumtuous, tender, fierce or aught else as suited the
moods of the message. Not bass--perhaps a rich tenor or
baritone. The English was free of either native or foreign accent,
perfectly pure of local or caste mannerisma, thus startling and
even uncanny at first hearing."

Crowley also made it very clear that it was not automatic writing, but that an actual voice was speaking to him...even though I doubt that ;)