The book often delves deeper into the mythology and lore surrounding Beren and Luthien. It might explain the significance of their actions within the larger context of the fictional universe. For instance, their relationship could be tied to ancient prophecies and powers that are explored in great detail in the book. In the story, while the essence of their relationship is there, it may not go as far in exploring these deeper mythological aspects. Additionally, the pacing in the book could be slower, allowing for a more immersive experience, whereas the story might have a faster - paced narrative to keep it concise.