[spekcular]

Issues with the license aside, there are substantial problems with the core algorithmic material of the book. The provided codes and approaches are often just plain incorrect.

Quoting from a review: "The authors of Numerical Recipes were not specialists in numerical analysis or mathematical software prior to publication of this book and its software, and this deficiency shows WHENEVER WE TAKE A CLOSE LOOK AT A TOPIC in the book [editor's emphasis]. The authors have attempted to cover a very extensive range of topics. They have basically found `some' way to approach each topic rather than finding one of the best contemporary ways. In some cases they were apparently not aware of standard theory and algorithms, and consequently devised approaches of their own. The MEDFIT code of section 14.6 is a particularly unfortunate example of this latter situation.

"One should independently check the validity of any information or codes obtained from `Numerical Recipes'...."

Source: https://www.uwyo.edu/buerkle/misc/wnotnr.html