Its been established that Wolf Team had a copy of Petersen's Field Guide to Cthulhu Monsters, as several enemies in El Viento are based on drawings in that book. From what I can tell, everything in Earnest Evans is something that Wolf Team came up with themselves. But there are a couple of references to the works of H.P. Lovecraft in the game... of which is the Necronomicon. In the game, the Necronomicon is described as an evil tome which has enormous power. Earnest is sent to retrieve it by President Coolidge, whilst Al Capone and Ullrich are also after it for their own plans. Ullrich believes he can use the book's power to influence the Nazis. It is likely that he also used the book to turn himself into a demon, and also use it to revive Hastur.

In the mythos, the Necronomicon was first mentioned in The Nameless City, a story published in 1921. The book wouldn't actually be named until it was mentioned in The Hound, a short story published in 1924. It is described as being a book of magic, containing information on how to summon the Great Old Ones, and stories about them. This ties into the game, as the Hastur Cult use the book to revive their God -at least, I think that's what they do, as nobody is ever actually physically seen using the Necronomicon at any point. In the game's ending, President Coolidge believes his country has no use for such a thing, and destroys it.

The other reference is the Elder Sign. The game has Earnest and Annet find this, although only Annet can remove it. It's a critical item as the slab it was found with has instructions on how to use a spell. At the end of the game, Annet is able to use the Elder Sign to cast this spell, sealing Hastur away. Later on in the ending, Annet throws the Elder Sign into the sea, where it quickly sinks and is never seen again.

In the mythos, the Elder Signs are used as a form of protection -those who yield them are able to drive off the Great Old Ones/Outer Gods, who seem to fear them. This ties in nicely with Earnest Evans, as the Elder Sign is used to thwart Hastur. However, the design of the game's Elder Sign is very different to what is described in the mythos, which state they had a design of a line with several branches, and a five pointed star. The game version seems to feature half of a human face, with a bird-like creature above it.