There's the classic "Godel, Escher, Bach" by Douglas Hofstadter. I won't try to give you a description or synopsis (as you can find countless others on the internet), but it's worthwhile to say the least. Long, and at times difficult, but worthwhile.
There are recent graphic novels about the lives of Bertrand Russell and Richard Feynman (respectively "Logicomix" and "Feynmann") that are more biographies than mathematics, but still very good reads.
There's also no shortage of recreational mathematics written by the late Martin Gardner. Gardner used to write a column called "Mathematical Games" in Scientific American. These can be found compiled into numerous anthologies.
There's one called "The Mathematical Experience" that gives insight into how mathematicians think, and what research mathematics actually consists of.
Finally, there's the short work of fiction "Flatland" which will take no more than an afternoon to read, but is another classic.
EDIT: I can also recommend the Orthogonal series by Greg Egan. These are science-fiction stories set in a universe with physics different than ours; among other things, light travels at varying speeds depending on its frequency. Again, these are sci-fi, and not mathematics, but Egan does a very good job of detailing and explaining the physics he creates, and sees-through their consequences in a manner that is very satisfying to a scientific mind.