Looking at the book in his hands, Eiro was actually quite surprised. Of course, he was hoping that this would work, but he hadn't expected to actually summon such a physical object. It was a tangible book, even if only given shape temporarily through the combination of different magics. Just sustaining it here outside of his body was constantly draining some of his mana.
But no matter what it was, he had just created a physical object that hadn't existed beforehand. He didn't store and retrieve something that always existed in the physical realm. Looking at it more closely, it seemed to be a physical object created through Arcane magic. Similar to what he created when using spells like 'Mage Hand', simply imbued with different magics, most notably, Knowledge magic.