The title of The Usual Rules refers to the fact that Wendy's life has been turned utterly upside down due to the events of 9/11—and the usual rules that she's come to rely on no longer apply.
Nothing about Wendy's life is normal or the same after her mother dies in the attacks on the World Trade Center, and she can't just go on like she always has. Her family changes, her perspective on the world changes, and even her home changes when she moves all the way to California to live with her biological father. For better or worse, the usual rules really don't apply anymore, and navigating this is the main struggle Wendy has to wade through.