It explains the mathematics describing drugs' passage into, through, and out of the body.

There are lots of books about pharmacokinetics, but this one is notable for being so clear, direct, precise, and readable. It explains the main causal factors, the not-so-obvious causal factors, the reasons behind peculiar notions in the field (e.g. a drug's "volume of distribution" might be much more than the volume of your body), and how to actually crunch the numbers. It doesn't just present results, it shows you the reasoning.

It's quite short, too: 170 pages. Nothing like those gigantic textbooks that say much less.