I'm afraid the ancient spellbook isn't anything special, it's simply a mass produced book of spells, or rather, was, that over 4000 years has become a rare find in the 6th age, especially after the Zarosian extermination.
Nowadays, only ancient Zamorakians, ancient Zarosians, the player and a rare few lucky adventurers have access to one such spellbook, although with Soran slowly rebuilding a Zarosian following, it may yet return to a more common practice amongst wizards.
The spellbook itself however is nothing divine, it's simply an amalgamation of spells developed by humans, Mahjarrat, Demons and Vampyres in Senntisten, using the advanced elements taught by Zaros via, presumably, the Pontiffs, which acted as tutors, magical researchers and philosophers alongside religious duties. In that regard, it was written the same way as the ordinary spellbook, through research, trial and error, not by an elder God.
Azzanadra is not yet a God, but as stated in CoM, he is close. He is still technically an ordinary Mahjarrat as of now, but perhaps strong enough to go toe to toe with a lower tier 6 God maybe.
Zaros may help him ascend in future however and we've already seen that Zarosianism is more a pantheon, with Orcus and Nex having their own temples, but ultimately still loyal to Zaros also, there is no reason Azzanadra cannot also have his own place as a Deity under Zaros.
I believe you may not be completely out of the ballpark however. Shadow magic specifically, may have a deeper connection to shadow anima, shadow realm and a possible artifact, but not the ancient spellbook itself.
17-Oct-2020 13:04:11