I find it shocking that "technical/instructive/informative" books are disallowed. I can't begin to fathom a reasonable explanation for that.

To be more helpful, a mathy algorithms text, like CLR (http://www.amazon.com/Introduction-Algorithms-Thomas-H-Corme...) might slip through the censors and be a good option--depending on whether your friend is comfortably enough with thinking about code abstractly.