Numerical Recipes in C is available free online. See Numerical Recipes Books On-Line. It's a great resource. The site says:
in reply to Re: Stirling Approx to N! for large Number?
in thread Stirling Approx to N! for large Number?
Thanks to special permission from Cambridge University Press, we are able to bring you, free, the complete Numerical Recipes books in C, Fortran 77, and Fortran 90 On-Line, in Adobe Acrobat format. Due to copyright restrictions, Numerical Recipes in C++ is not available as part of this free resource.
The factorial representation is available at this PDF (after a I chose LANL as the mirror site).