I watched the whole series before picking up the books, and the shock was real. The tone is probably the biggest shift. The show is very much a CW teen drama with supernatural elements—lots of romance triangles, quippy dialogue, and fast-paced melodrama. L.J. Smith's original novels, especially the first few, feel more gothic and introspective, with Elena as a blonde, popular queen bee who's more calculating and obsessed with status at the start. The Stefan and Damon dynamic is still central, but their backstory and motivations are altered significantly for TV. I found the books more atmospheric, but the plot can meander in later entries.
Honestly, I prefer TV Damon. Book Damon is cruel in a less charismatic, more petulant way initially, while Ian Somerhalder brought a vulnerability that made the redemption arc work. The show also introduces a massive amount of original mythology and characters—the entire Originals storyline, for instance, which spawned its own spin-off. The books have a different set of villains and lore that never get as expansive. If you're coming from the show, the books might feel like a familiar yet oddly different blueprint.