With the hypercompetitiveness of modern scientific careers, scientists who take time off to write popular expositions are looked down upon. The problem is not Malcolm Gladwell, the problem is the insularity of modern scientists.
W.W. Sawyer, What is Calculus About? and Mathematician's Delight
Courant and Robbins, What is Mathematics?
Hogben, Mathematics for the Million
Steinhaus, Mathematical Snapshots
Ivars Peterson, The Mathematical Tourist
Davis and Hersh, The Mathematical Experience
Polya, How to Solve It
Huff, How to Lie With Statistics
McGervey, Probabilities in Everyday Life
Raymond Smullyan: The Lady or the Tiger, Alice in Puzzle-Land, others
Anything by Martin Gardner. I happen to have picked up Mathematical Magic Show and Mathematical Circus, but I'm sure there are many other collections.
I also recommend cryptography stuff. David Kahn's The Codebreakers is not really a math book, but it is awesome and it stars mathematicians, as does Simon Singh's The Code Book. You could read Schneier's Applied Cryptography.
This is HN, so I would be remiss if I didn't point out that you can learn a lot of fun and useful math by reading SICP, Knuth, or any good algorithms book.
If anybody out there knows a good, spirited statistics book addressed to someone who knows calculus, tell me. I keep planning to go through Fundamentals of Applied Probability Theory but I never get around to it; see "Related Resources" here:
http://ocw.mit.edu/OcwWeb/Electrical-Engineering-and-Compute...
Having said all of that: I have a Ph.D. in physics/EE, so I've got to tell you, if you haven't tried calculus you haven't lived. ;) I'm not sure how to go about learning calculus in a fun way for a mathematician -- I took fairly standard first- and second-year college courses in calculus and physics and learned it that way. The folks on Amazon seem kind of enthusiastic about Spivak:
http://www.amazon.com/Calculus-Michael-Spivak/dp/0914098918/...
- The last book is not really that advanced, It can be understood by most with an understanding through high school algebra II.