Coffee tastes bitter but it sure does have caffeine in it, which is a win in his book.
It means he can vaguely keep up with Aang's monologue. Well, not the tetris, he has no clue what that is other than a ?game? of some sort, and in general his knowledge of computing is a thousand years out of date.
Also this is exactly the sort of person who would teach the teenagers to make bombs. What other kind of person did he expect. Aang is the distillation of the sort of person who would volunteer to do that.
He scribbles down the names of the books on a napkin, to look at when he has time to read things other than 'An Introduction To Particle Theory' and 'Models of Early Human Migration.'