Who is Klaus in the books?

In an interview with Entertainment Weekly, Julie Plec had this to say about Rose dropping Klaus’ name in the final moments of episode 208:
The mystery of Klaus and when we’ll see Klaus is going to be the same question of ‘When will we see Katherine?’ from last season. We’re gonna learn a lot about him and be hopefully very afraid of him before we ever lay eyes on him. That’s gonna be a nice, big, long healthy tease. It’s so funny, the way we dropped the bomb of his name last episode [Rose said The Originals, the oldest vampires, will be coming for Elena, and they'll be doing it for him, Klaus], Kevin and I were laughing. We’re like, the book fans are gonna lose their freakin’ minds in this moment, and the rest of America is gonna go, ‘Huh… Alright. Who’s that?’ [Laughs] But it’s one of those moments that you’re like, I don’t care because I know the million book fans are gonna be so excited that it’s worth it.”

And she was right! In the Vampire Diaries book series, Klaus is actually Katherine’s maker and, as is par for the course with the TV series, they’ve switched it up to keep book fans guessing. In the show, Katherine essentially makes herself, after tricking Rose into giving her blood; her history with Klaus is now much, much different. In the books, Klaus is the Big Bad of book 4, Dark Reunion, come to wreak merry havoc on Fell’s Church, Virginia (because he was apparently quite bored). Tall, Nordic-looking, powerful, charismatic, and creepy as hell, the Klaus of the books claims to be carrying a bronze ax in his earliest memories. He makes passing references to the fall of Rome, fighting with Alexander’s army, and being in the Trojan War, so you know he’s old. He also brags a lot, but what makes him scary is that his follow-through on said boasts is excellent. Also, he’s so insane he’s wrapped back ’round to sane again, if you can imagine that. (L.J. Smith once stated that she based the character on Rutger Hauer in The Hitcher, possibly one of the greatest horror movies of the eighties.) He also has a penchant for quoting poetry (Kipling, Auden, Housman, and Roethke) and has a soft spot for fifties music (see if you can ever listen to The Spaniels’ “Goodnight, Sweetheart, Goodnight” the same way again after reading Dark Reunion).