Man, this question hits right in my sweet spot. The transformation and lore bits are my favorite parts of werewolf fiction, and I think a lot of books treat the transformation itself as just a cool visual effect, but the ones that dig into the physical and psychological agony really stand out. Steve Morris's 'The Wolves of London' series comes to mind—it’s more horror-leaning, but the transformations are visceral, painful, and tied to this deep, ancient lore about lycanthropy as a curse passed through bloodlines. The lore isn't just pack hierarchy; it's alchemical, rooted in medieval grimoires, which feels fresh.
Another that does lore incredibly well is Maggie Stiefvater's 'Shiver'. It frames the change as temperature-dependent, which is such a simple yet brilliant twist on the lunar cycle, and the lore builds slowly through journal entries and local legends. For pure, unadulterated focus on the transformation process itself, you can't beat the short story 'The Werewolf' by Angela Carter. It's a dense, gothic, fairy-tale retelling where the transformation is almost a metaphor for puberty and sexual awakening, layered with old-world superstition. Most modern shifter romances breeze past the actual change to get to the mate-bond drama, which is fun, but these books make the metamorphosis the central, unsettling mystery.