The Necrotelicomnicom is another reference to the Phonebook of the Dead (see the annotation for the dedication of Equal Rites), but is also a pun on the evil book of the dead Necronomicon, used by H. P. Lovecraft in his Cthulhu stories.
Bel-Shamharoth is an Elder God of the Discworld we already met in ‘The Sending of Eight’ in The Colour of Magic. C’hulagen is obviously made up out of the same ingredients as C’thulhu, and the Insider refers to the unnamed narrator of Lovecraft’s The Outsider