As it turns out, very different, while still very similar. To me, The Vampire Diaries books are super-boring, tiring and so like the Twilight series that I'm surprised they are not the same story. Book Elena is such a pain in the butt and so very "I will never leave you Stefan" and "I will die for you Stefan" that if I weren't determined to read the books, I may have packed them away in the donations box. The TV show is, in contrast, a lot more mature (yes, there's a lot of skin too), intriguing and the characters are so much more interesting and three-dimensional.
Still, the book served its purpose of letting me devour a book. Sometimes brainless reading is just what I need. It was good enough that I kept coming back to it, but not enough that I would consider it a substantial reading. I probably will continue with this series - the more I read this series, the more I'm thanking the TV show producers for making it very different from the book.