The Arcanomicon is the map of all of the magical ley line intersections in the world of Azeroth. Gifted to Malygos ages ago by the titan Norgannon, the Arcanomicon has been continuously updated and revised by the blue dragons over the long years of Malygos's seclusion. Malygos planned to use the Arcanomicon to locate and tap into the ley lines and divert the magical powers that course beneath the earth to his home base in Northrend, the Nexus.
Only a projection of it is ever confirmed to be seen. It is not certain if the Arcanomicon exists as an object in the traditional sense.
Given the description of the titans in Chronicle, Norgannon my not have directly given the Arcanomicon to Malygos. The titans had already departed Azeroth by the time the Aspects were empowered. It's very likely that the keepers who channeled the Pantheon's power into the Aspects gave Malygos the Arcanomicon instead.