When Chick writes about Norman in his journal, he uses a variation of the words that the psychiatrist who questions Norman after his arrest towards the end of Psycho (1960) uses when telling the police about his "Mother" persona.
The Convergence of the Twain is a poem by Thomas Hardy, published in 1915. The poem describes the sinking and wreckage of the ocean liner Titanic.
The book, "The Lost Art of Mummification" that Caleb finds when he arrives at the Bates house looking for Norma is a reference to the condition of her corpse in Psycho (1960). Feeling guilty over her death, Norman mummified her corpse after digging it up.
The Convergence of the Twain refers to two things becoming one. In this case it is Norman and Norma.
The character of Sam Loomis, played here by Austin Nichols, was originally played by John Gavin in Alfred Hitchcock's Psycho (1960).