As for why this Shadow Codex had been used to prop up a table leg... perhaps Dumbledore's SAN was too high for him to see what was unusual about the book.
But that worked out perfectly. Otherwise, it might not have fallen into Kane's hands so easily.
Kane let the shadows erode and melt the Shadow Codex, absorbing it into his own shadow. Almost instantly, he felt his shadows become more solid, more active, and more obedient.
Previously, he had been able to make a copy of Hermione's shadow like crafting a marionette, directing it bit by bit to comfort Hermione. Now, he could directly create shadow clones with basic mobility, just like Maxwell had.
Furthermore, he no longer had to go through the trouble of using Transfiguration and Shadow Erosion to create hostile creatures shaped like those from the Eternal Realm to attack his enemies.
With the Shadow Codex, he could directly create and summon all kinds of Shadow Monsters to attack his enemies, cutting out the Transfiguration middleman and its markup.
Of course, the Shadow Codex definitely did far more than that. Maxwell had studied it for so many years and had only barely mastered four spells within it. For Kane to gain two powerful methods the moment he got it was already excellent. If he wanted to further uncover its knowledge and essence, he could only keep studying diligently.
"Mr. Wizard? Can we leave now?" The goblin standing outside the vault door, with his back turned so as not to look inside in accordance with the rules, asked softly.
"We can leave."
Kane pulled out a stack of Galleons from Little Pumpkin's mouth, used them to level the crooked bookshelf again, then swiftly left the vault.
After locking the door, he followed the goblin back to Gringotts.