teaspoons and tablespoons are metric - one teaspoon is 5 ml; one tablespoon is 15 ml. I'm not sure how useful it is having a volumetric unit instead of a mass[1] unit; but that's one thing I like about US cooking. "About a cup of this; about to cups of that" - it's all nice and intuitive. I know roughly what 500 g of flour is, or sugar, but 80 g of butter is tricky.
[1] sorry if I'm using the wrong term. Friendly corrections welcomed.
There's loads of references in that book to knobs of butter and tablespoon/teaspoon measurements.