Frankly, I think a lot of readers get hung up on the 'power levels' aspect and miss the point. Norman's development is less a straight upgrade path and more a deepening entanglement with the cost of his magic.
Early on, his power is clumsy, fueled by raw desperation and academic curiosity. He reanimates a mouse, then a cat, and the descriptions are full of revulsion—the smell, the wrongness of it. He's a scholar, not a warrior. The shift happens around the midpoint, during the siege at Harrowgate. He's cornered, and instead of just raising individual corpses, he unconsciously taps into the latent death-energy of the battlefield itself. The ground literally shifts. But the aftermath is key: he's catatonic for three days, haunted by the echoes of every soldier he used.
Later development gets psychological. He learns to 'listen' to the dead, which is terrifying and gives him information but also fractures his sense of reality. His ultimate 'power-up' isn't a bigger spell; it's a horrifying pact where he allows a lingering spirit partial possession in exchange for precision. So his power 'develops' by becoming more efficient and potent, but at the direct expense of his humanity. By the end of book three, he's arguably the most powerful character in the region, but he's also a gaunt, spectral figure who can't bear to be touched by the living. The magic doesn't just change what he can do; it changes what he is.