Giovanni Battista della Porta (1538-1615), also known as Giambattista Porta, was an Italian physician and poet interested in sorcery. The work mentioned in by Lovecraft is the De Furtivis Literarium notis, a five-part poem, often reprinted and considered one of the early masterpieces of cryptography. He also wrote "Magia Naturalis" or "Natural Magick," a work in twenty books which deals mostly with alchemy. Basic Books of New York published an edition of this in 1957. He founded the Academy of the Secrets of Nature (Academia Secretorum Naturae) in Naples.
("The Dunwich Horror")
See also: De Furtivis Literarium Notis