Spoiler!
His soul is not human and eventually it will affect his human body allowing him to use his full abilities as a human including transforming his human body. Like the title says he is not the lackey, his the eldritch god itself who let it's consciousness be born as a human while leaving his main body in the world he transmigrated to. All the books are knowledge he gathered as an eldritch god and he'll eventually find out his true identity(he's also the god the elf king fought and lost to)