Tree Tutorial 8 Modal Trees
Colin Howson,  Logic with trees Chapter 12 Section 2
The Howson  does not expand on modal logic (and modal trees) so a text like
Rod Girle  Modal Logics and Philosophy
would definitely be a help here.
[Modal logic is a vast area, what is being presented here is the briefest of glimpses through the shop window (a book like the Girle would help you go further).]