Math as easy as 1-2-3 Teach Yourself Basic Mathematics gives you the confidence to handle the math you encounter in daily at work, while shopping, and in home life. It features puzzles and games to make numbers interesting and to sharpen your skills.
A nice study guide for middle-school-level math written for adults. Includes lots of explanations on why the principles are true and has several brain teasers and logic puzzles. Is written from a British perspective, so Americans might find some examples hard to understand due to the framing.