Dimensional Analysis

This is one of my favorite subjects - if only because it lets you solve physical problems without knowing much physics. The best beginning book is "Of Size and Life" by Tom MacMahon (I wonder if this is still in print?)

Oddly, despite all the fuss about radians and degrees, angles are actually dimensionless quantities (being ratios of lengths, in fact: the arc of the circle cut out by the angle, divided by the radius of the circle).