The Little Algebra Book is a short, colourful and beautiful introduction to basic linear algebra. Written by Colin and designed by his brother, Stuart (an award-winning designer at The Chase in Manchester), it can be yours for just £3.70 (if you're in the UK) or £4.75 (rest of the world), including postage.
From Goethe having a go at the French to Neil Armstrong whipping up a nerd-frenzy, people have been saying witty and thought-provoking things about maths since the year dot. I've collected more than 100 of these and turned them into an e-book for your reading pleasure. It's about £3 for the Kindle.
Need to figure out how to resolve forces so you can rock your Mechanics 1 exam? You'd better get hold of this before the Russians figure out what's going on. You can pay what you like for it.