Lara Jean and John Ambrose's friendship-turned-fliration grew a lot more steadily and intensely in the book. They wrote letters, they worked together in the Assassins game, and they put on the USO party (not the Star Ball) at Belleview. John Ambrose was a lot more open about his feelings in the book, but was ever the gentleman.
The two shared a kiss in John Ambrose's car rather than at the Belleview party in the book. And unlike the movie, Lara Jean didn't have an immediate epiphany that she wanted to get back together with Peter. Instead, she and John Ambrose stayed close until later when she made her choice.