You'll probably get a dozen answers just listing the Salvatore brothers, but the book series actually has a way bigger core cast than the show, and Elena's the sun everything orbits around. Forget the TV version's Elena Gilbert; book Elena is blonde, popular, and a bit more of a schemer at the start, which makes her dynamic with the vampires way different.
Stefan and Damon are there, of course, but their backstory and personalities shift from the novels to the TV adaptation. Bonnie is a huge part of the books as Elena's best friend and a seriously powerful witch, way earlier on. Meredith, another friend, is a pragmatic future lawyer type who brings a great grounded energy. And then there's Matt, Elena's ex, who's a much more substantial character in the books.
They all get tangled up with other figures like Klaus, who shows up later as this ancient, terrifying hybrid origin story. I always felt the books had a more sprawling, high school drama meets gothic horror feel, with Elena's friend group being just as crucial as the vampire romance.