There’s only a few quotes from Spates in the IDW canon so I can just list them here but they’re all in the wiki along with scant glimpses of the Idulnas pages (https://ghostbusters.fandom.com/wiki/Spates_Catalog)
In the first arc, pages literally appeared and text filled itself in Spates Catalog, and an entry on Idulnas appeared, “And so the third minion of Gozer, Idulnas, came into being and his power eclipsed that of Zuul, and it surpassed that of Vinz Clortho. Idulnas would, through his power, allow for a new selection of the Traveller’s Form. And through his power, the world would drown in the tears of its people.”
In Volume 2, Ray and Kylie used it among other books to look up instances of recurrent child hauntings and supernatural abductions in the last 50 years when they did bogeyman research. Or alternatively, you could always through in Grundel.
Then in Ghostbusters 101 #5,
“A more sensationalized reference than its contemporary – Tobin’s - the Spates Catalog is a compilation of an annual periodical published by T. Watson Spates from early 1901 to 1935.”
“It was billed as a “compendium of nameless horrors” (a slight misnomer since, in fact, it gave names to most of the horrors of which it spoke) and was a favorite of both Harry Houdini and H.P. Lovecraft.”