I don't have the Mystic handbook yet (but will be getting it) what is heartwood? What is special about it?

Fluff-wise? It's culturally significant to Tholothians, a connection to the communities they come from and their homeworld of Tholoth.

Mechanically? Hilts made from it connect their attacks more easily, but increase their encumbrance and are easily damaged. There's also the Heartwood Blaster (rifle) that provide advantages for strain.

Honestly, the book made me fall in love with Tholothians and their hometrees, and if I ever get the chance to play a character myself, it'll probably be a Tholothian Guardian.

I know that in Clone Wars Cartoon the Wookie padawan made his lightsaber out of Heartwood and I have wanted my character to make his out of the same material because while not Tholothian he is from a world untouched by tech and I feel that a Heartwood hilt fits him better then any other material.

But until this book it was just going to be a fluff Heartwood saber