Via Greg Mankiw, I just found out about Oxford's Very Short Introductions series. Greg has high praise for Mathematics: A Very Short Introduction. When I looked it up on Amazon, I found numerous other books in the series recommended to me: Particle Physics, Cosmology, Music, Economics, Theology, The Brain. There were about 20 that immediately sounded interesting.
As I explored more I found more and more books in the series. Oxford's Very Short Introductions site indicates over 150! Damn, almost every single one of them looks interesting.
As well as individually, they sell them in boxed sets like the "Basics Box" (Philosophy, Maths, History, Politics, Psychology), the "Brain Box" (Evolution, Consciousness, Intelligence, Cosmology, Quantum Theory) and the "Thought Box" (Hegel, Marx, Nietzsche, Schopenhauer, Kierkegaard).
As is generally the case with these sorts of series, I'm guessing the quality varies a lot. Still, it's hard for me to resist buying a large proportion of them right now :-)