For algo/datastructures, CLRS (http://www.amazon.com/Introduction-Algorithms-Thomas-H-Corme...) is the gold standard.

For OS's, Tanenbaum (http://www.amazon.com/Modern-Operating-Systems-Andrew-Tanenb...) is popular.

'Math' is broad - if I can recommend only one book to cover all of Math I'd probably say 'The Road to Reality' (http://www.amazon.com/The-Road-Reality-Complete-Universe/dp/...). More practically (for the subset of math most programmers are likely to care about), you'll do fine with one good discrete math book and one linear algebra book. Throw in one each on Stats, Abstract Algebra, Calc (up to ~diffeq), and Real Analysis (in roughly that order) if you're a bit more ambitious ;-)