Numerical Recipes is a classic of Scientific Computing.
Download here the whole book in ps or pdf.
A little story. Older versions are freely available here. If you are lazy, like me, and you get bored of saving every single file by hand, you can use this fantastic Firefox plugin. Or download them zipped here.
If you are lazy, like me, and you get bored of having to open a new file for each chapter, you can merge them in one monolithic file.
If you are really lazy, like I wish to be, you can use my job and download the complete book in ps or pdf straight away.