I know this is a little late for Christmas, but nevertheless, I have a few (some of which have already been mentioned) books I've read that I've quite enjoyed. For the sake of brevity, I'll let you search the titles on Amazon for reviews and better descriptions.
Title: Everything & More: A Compact History of Infinity Author: David Foster Wallace
Title: The Mathematical Experience Author(s): Philip J Davis & Reuben Hersh
Title: One, Two, Three...Infinity Author: George Gamow
Title: Pi in the Sky Author: John D. Barrow
Title: Prime Obsession: Bernhard Riemann and the Greatest Unsolved Problem in Mathematics Author: John Derbyshire
Title: Strength in Numbers Author: Sherman Stein
Title: e: The Story of a Number Author: Eli Maor
Title: A History of Pi Author: Petr Beckmann
Title: Nature's Numbers Author: Ian Stewart
Title: Mathematics: The Science of Patterns Author: Keith Devlin
Title: Zero: The Biography of a Dangerous Idea Author: Charles Seife
Title: How to Enjoy Calculus Author: Eli S. Pine (Not really a "popular" book, per se', but still pretty good)
Title: How to Think About Weird Things Author(s): Theodore Schick & Lewis Vaughn (Not really about mathematics, but not so far out of the way that you wouldn't enjoy it if you also enjoy mathematics)
