A dread tome of legend. The Mad Poet's Book in the Dimension of Dreams is one version of it, but a copy apparently exists on Golarion in the Real World as well.
According to the Mad Poet, it contains the location of Neruzavin, along with many “useful” rituals.
In Chapter 153, Pandora recalled the lore she knew about the Necronomicon:
The blasphemous tome, containing hundreds upon hundreds of pages of dread truths about the Dark Tapestry and the Elder Mythos, was supposedly written on another planet entirely, by a man named Abdul Alhazred, about whom there are countless stories and legends, none of which can be verified or proven.
Whatever happened to the man himself, it is known that his book made its way into the Dreamlands. And some say that ever since the Necronomicon found its way into the Dreamlands, every planet in the universe that supports lifeforms able to dream has had a copy of the tome make its way there. Someone will dream of finding it in the Dreamlands, and then will be unable to stop dreaming of reading it, night after night, and then every day in the Real World they will transcribe what they have seen and learned, and eventually, on every planet, this lone dreamer will skin people alive to make a new cover, pour their soul into the wards and the bindings, and then…. a new Necronomicon is born. One for every world. And thus its dread truths find their way into new civilizations, and its madness spreads. On and on and on.
Golarion’s copy of the Necronomicon was translated into Necril, and intended as gift for Tar-Baphon, the Whispering Tyrant, nearly a thousand years ago. It was created by a necromancer named Geir, long believed to have been destroyed in the Shining Crusade—probably erroneously, because he became a demi-lich. The paladins who thought they’d put him down permanently took possession of the book and sent it far away, to the city of Zimar, a major Taldoran stronghold on the border with Qadira.
Of course, the scholars there attempted to figure out what the tome was. This proved challenging for several reasons. They quickly learned that the Necronomicon’s wards were unlike any they’d ever encountered.
It is divination-proof. Any attempt to use any type of divination magic to learn about it, determine its location, to view it, or any part of it, or anyone possessing it, not only automatically fails, but the scholars who tried to do so were instantly driven mad.
It is teleportation-proof. Any attempt to use teleportation magic to change the book’s location in space results in the book being left behind and the spellcaster ending up in unknown location. Or, at least, everyone who ever tried teleporting it was never seen again, and definitely not at the spot to which they’d intended to travel.
It cannot be destroyed, by any verifiable method. There are those who believe that finding and destroying Abdul Alhazred might possibly allow for the book to be effectively destroyed, but that any damage taken by the book would most certainly draw the direct attention of its suspected muse, Nyarlathotep, the Crawling Chaos.
Unable to divine information about it, and unable to destroy it, one of the scholars read it. By the time he was done, he was unrecognizable to his colleagues. The quiet, gentle, devout servant of Shelyn had become an insane worshipper of Nyarlathotep who murdered dozens of innocent people before the paladins could put him down and stop his reign of terror. No one knew what to do with the book, so they put it in a secure vault and decided to deal with once the Shining Crusade was over. However, by the time that happened, the few who had known of it were dead, and over time the Necronomicon was forgotten.
Three hundred years later, Qadira and Taldor were at war, and the city of Zimar was captured and sacked. Whatever happened next to the Necronomicon likely happened then, around 4079 A.R. Treasures of all kinds—great works of art, weaponry, and books—were liberated from the city and sent south. Where exactly the Necronomicon ended up is not a known fact, but certain scholars—like Miacknian Mun—have spent lifetimes attempting to track it down, investigating mysterious murder sprees and unexplained events around the world, hoping to uncover a whisper of the tome’s current whereabouts.
In Chapter 166, the party learned that Mun believed the Necronomicon was being held in a location known as the Mysterium.
Count Lowls’ theft of the Necronomicon from the Mysterium was witnessed in Pandora's psychometry reading of the Ring Gate during Chapter 183.
First Reference: Chapter 150 (Part 2)
Other Notable References: Chapter 153, Chapter 166, Act IV Interlude Part 1, Chapter 183