Thanks for buying the book.
You're right. Here's the sort criterion:
So you could, in theory, control it by setting the menuindex on all the resources. In fact, it might work to just sort the tree by pagetitle.
You could also change that code in
core/model/modx/processors/element/tvs/renders/mgr/input/resourcelist.php (line 50) to this:
I don't think it would have any adverse side effects, but I'm not positive.