One book I'd recommend is "1089 and all that" (http://www.amazon.com/1089-All-That-Journey-Mathematics/dp/0...).

I've bought copies for friends and family whose maths backgrounds range from school to degree level and they've all enjoyed it.

It's not a textbook, more a conversational account of a few topics the author has found interesting through his life from a young boy to an Oxford don. It's very readable and the author's delight in maths and problem solving shine through