Q: Does anyone know where the source code from books like Numerical Recipes in C, Plauger's The Standard C Library, or Kernighan and Pike's The UNIX Programming Environment is available on-line?
A: Books containing large quantities of potentially-useful source code, including the ones mentioned in the question, usually make explicit mention of the availability of source code and policies for its use. Published source code is copyrighted, and may generally not be used, or especially redistributed, without permission (and perhaps a few restrictions, though presumably the publisher doesn't mind your typing it in for personal use). Often a diskette is available from the publisher; also, many publishers have set up ftp sites and web pages.
Some of the routines from Numerical Recipes have been released to the public domain; see the Numerical Recipes website for details.