Most likely the author of The Seven Cryptical Books of Hsan.

In some cases a manuscript is named after its author, a religion or philosophical ideal, or the place where it was found.  Many times it is named after the person who owns it and makes it famous (such as the Vornich manuscript).  It is likely that, in this case, it is the collected works of a single anthor, a common habit in early times: see the Three Books of Occult Philosophy of Agrippa, for example.

[The Dream-Quest of Unknown Kadath - H.P.L.]

See also: The Seven Cryptical Books of Hsan