First off, the core lore is basically unrecognizable. The 'Vampire Diaries' books started as a fun, pulpy supernatural soap in the early 90s. Elena is a vain, popular blonde who schemes her way through high school, which is a far cry from Nina Dobrev's more melancholic portrayal. Stefan and Damon are centuries older in the show; in the books, they're Italian Renaissance-era, I think? The mythology is less about doppelgängers and ancient bloodlines and more about these psychic powers some vampires have, like telepathy.
Honestly, the book plots get wildly off the rails with time travel, kitsune demons, and characters getting trapped in different dimensions. The show streamlined a lot of that into a more cohesive, if still messy, mythology. I have a soft spot for the books' chaotic energy, but the show's characters, especially Damon and Bonnie, ended up with way more depth. The books feel like a fun, guilty pleasure relic now.