One thing to keep in mind is that the tone for Tress is far different from any of his other books. It's much more... whimsical. That said something that's a bit more of a step towards normal Sanderson but still a bit of that tone is Yumi and the Nightmare Painter. It's the same character narrating, though it isn't quite as light hearted.I just listened to my first Sanderson book - Tress of the Emerald Sea - and freaking LOVED it. I've seen ideas about which of his books to try next, but what does bookEra think?
Easiest next step over all would be to dive into Mistborn or Warbreaker.