Do you know what type of things interest them? What is their area? Are you looking for a general book, or perhaps a biography of a particular mathematician? Some more information always helps. – 2012-01-02
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Maybe this thread helps: http://math.stackexchange.com/q/31058/ – 2012-01-02
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"God created the integers", a collection of translations of classic texts assembled by Stephen Hawking (or published under his name, anyway), is good in my view. (At least, I am a mathematician who received it as a gift from a family member one Christmas, and I was very pleased to get it.) – 2012-01-02
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I had two copies of an book on the history of computers and the effects of computers on our lives. I eventually just gave the extra copy to a computer science Ph.D. student, guy who played soccer with me. He said "You're giving this to ME?" meaning that there was little chance a book I had picked out in his topic had much to tell him. So bear that in mind. Something in the nature of a diversion might be better, and less likely to already be owned. – 2012-01-02