Well, it depends on what you want to get out of it. I could recommend many, many books, but some of them are a lot "deeper" than others. I would give The Intelligent Investor, for instance, to someone that was interested in falling asleep as quickly as I'd give it to someone who was interested in investing.
That said, my "light" list:
-The Millionaire Next Door
-Anything by William Bernstein(Four Pillars, Intelligent Asset Allocator, The Investor's Manifesto[covers risk/reward exceptionally well])
-Stocks for the Long Run(this was my first book)