Geoffrey of Monmouth wrote the “Prophesies of Merlin,” the History of the Kings of Britain, and “The Life of Merlin.”
“The Prophesies of Merlin” was distributed before it was incorporated into the History of the Kings of Britain. A copy of the History was in the monastery at Bec in Normandy in January 1138.
“The Life of Merlin” is the last work Geoffrey wrote.
I recommend the following edition of the History.
Geoffrey of Monmouth: the History of the Kings of Britain. Latin text edited by Michael D. Reeve. Translated by Neil Wright. 2007, The Boydell Press, Woodbridge. ISBN 978-1-84383-441-0
I use the following online version of “The Life of Merlin.”