If you're not familiar with the book, it's basically your average bedtime story for a child. Except it also has some explicit pleads from the parent reading the story begging the child to, well, go the f*ck to sleep.
What makes Jennifer Garner's reading so perfect is her total mom persona. She has, after all, played several of those perfectly poised and freakishly attractive moms Hollywood brought us all up on. It's incredibly satisfying to hear her dainty voice shout, "HELL NO you can't go to the bathroom!" to a hypothetical child.
Jennifer herself has three kids, but all I can picture is her reading this exact story to Ben Affleck, back in the day.