An undergraduate combinatorics textbook by Ronald Graham, Donald Knuth and Oren Patashnik. Beautifully typeset, using TeX. The title is in contradistinction to abstract mathematics; it has nothing to do with civil engineering.

Concrete Mathematics is just that -- for computer science. The book is strictly on a need to know basis. It just shows you that what you need to know of mathematics to do (some) computer science can be very beautiful.

Particularly noteworthy are the sections on generating functions. These are always useful in CS; __Concrete Mathematics__ shows advanced, beautiful techniques, that are 100% concrete.