Display booktree in block

I'm sure this a very basic question, but I've been searching and can't find how to do it.
What php snippet would go in the block in order to display the booktree?

Actually the question is not specific to the booktree module, but just php and drupal modules in general.

There is an InsertNode module (http://drupal.org/project/InsertNode), but that doesn't work because the page produced by the booktree module is not a node.

Thank you.