Ok.I know I'm going to sound like one of those whiney people that don't even try, but I've completed all 4 books and gone through them again after banging the snake chick. When I'm in book 2 for the 2nd time I can't leave to go to anywhere. I only find Mai and Azula in the shower.If I go back again for the 3rd time all I can do is have the orgy at the Southern camp.
How do I get to Book 5 and Ember Island.
As far as I know, you have to go through Book 4's Love Route and go through a portal to the Spirit World. Make a choice, have sex with the snake lady, or the spirit chick. Depending on the choice you make during this sequence, you complete the story, get an ending, and begin Book 5. From there you should gain access to Ember Island with the elderly versions of Katara, Toph, and Azula as well as scenes with Izumi, Lin, Suyin, and Opal.
Make sure you save before entering the portal just in case.