oooc: that means look at the "knowledge arcana" ooc above.
Okay, you guys have been quiet, so I'm moving this along: the party gets all their magical treasure and sits in the secret base for some time. Malacarus and Hedron discover exactly what they remember about "Astral Books", and decide to share it with the party.
In short: some tomes are so "primal" that their true form exists on the Astral Plane, with physical copies made by seers throughout histories being transcriptions of the original, Astral version.
It sounds like the Astral Necronomicon would be a version of this: a powerful tome with the source of the knowledge on the Astral Plane.
You should go to the Astral Plane, find the tome, and learn more about the foes you'll fight; and maybe how to stop them.
The party, now super-charged with their new suite of items, has a chance to test themselves once more against the world. What horrors, however, could they face in the Astral Plane?