The way I see it, each book has its own unique feel and atmosphere, which cannot be compared to the others. Each book has its own charms, highs and lows, which lose their meaning when viewed collectively, and personally, I like it that way. Makes the game feel more dynamic instead of repeating the same concepts over and over.
In Book 1, your goal is to simply woo Katara, and nothing else.
In Book 2, you try to balance your relationships with your crazy sister, your gloomy girlfriend, and TyLee being TyLee, all the while trying to regain your memories, acting as a prince and leading the fire nation army in the war.
In Book 3, you try to have a meeting with the Earth King (although this gets put way down in your list of objectives), after you get evicted from Ba Sing Se. You build a small village with the help of Toph and Katara, while trying to overthrow the secret organization Dai Lee, which is controlling the politics of Ba Sing Se from the shadows, and controlling the Earth King as well. You explore the underground maze of Lake Laogai, free those who are captured, provide them shelter in your village.
In Book 4, you help the present Avatar in her darkest times, help her get back on her feet, visit old friends from the previous books, hook up with literally every girl in sight, all the while trying to stop the Earth Kingdom from starting another war.
P.S. I'm talking about the Love route, haven't played the Slave route yet (not a fan of mentally torturing or breaking, unless the game doesn't allow otherwise (still feel bad tho) ).