O'Rourke [O'R94] is perhaps the best practical introduction
to computational geometry available, because of its
careful and correct C language implementations
of the fundamental algorithms of computational geometry.
Fundamental geometric primitives, convex hulls,
triangulations, Voronoi diagrams, and motion planning are all included.
Although they were implemented primarily for exposition rather than
production use, they should be quite reliable.
The codes are available from http://grendel.csc.smith.edu/ 
orourke/.