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Concrete Mathematics: A Foundation to Computer Science by Graham, Knuth and Patashnik.
From the fatbrain page:
Concrete mathematics is a blending of continuous and discrete mathematics. "More concretely," the authors explain, "it is the controlled manipulation of mathematical formulas, using a collection of techniques for solving problems." The subject matter is primarily an expansion of the Mathematical Preliminaries section in Knuth's classis Art of Computer Programming, but the style of presentation is more leisurely, and individual topics are covered more deeply.
