There's no hyperbole in the title - this truly is complete, and makes a very interesting read even if you're not an artist, but simply want to dig deep into facial expressions.
It goes over why and how facial expressions are as they are, describing the muscles that push and pull on our faces when we show emotion. I wasn't expecting to learn this much, to be honest, such as that there's one muscle on our faces that works involuntarily, and only in one expression (when we're sad). My other favorite bits were the comparisons between genuine smiles and fake smiles: in short, in a fake smile your mouth looks very boxy (and eyes are usually neutral-ish), and the examples cracked me up because they looked like pretty much every (American) kid's first day at school photos that get posted on social media, haha!
There are also cool comparisons between two drawings of a face, where only one section is altered (such as the wideness of eyes, or a fold near the mouth is removed), and how that telegraphs a totally different kind of emotion.
A great reference book!