The product of a unique collaboration among four leading scientists in academic research and industry, Numerical Recipes is a complete text and reference book on scientific computing. In a self-contained manner it proceeds from mathematical and theoretical considerations to actual practical computer routines. With over 100 new routines bringing the total to well over 300, plus upgraded versions of the original routines, the new edition remains the most practical, comprehensive handbook of scientific computing available today.
"The research worker, in his efforts to express the fundamental laws of Nature in mathematical form, should strive mainly for mathematical beauty. He should take simplicity into consideration in a subordinate way to beauty ... It often happens that the requirements of simplicity and beauty are the same, but where they clash, the latter must take precedence." ~ Paul Dirac
I use a lot of books for reference in my research. Many books of this type lend themselves to not direct reading but more as reference to flip through for specific needs. This book does both those things. Over the past year I cannot think of a book which I have both consulted more, on a wide array of topics or enjoyed more as reading on topics of general interest. Even if you're not writing in C, due to C being the progenitor language of most common languages used today, the code in this book makes for great sudo code.
I found the sections of pade approximants, moments of distribution (statistics chapter) and the section of random numbers to be both well written and adequately explored.
"What we know is not much. What we do not know is immense." ~ Pierre Simone Laplace
It was a great source for code to do many numerical programming tasks, such as integrals and derivatives, finding roots, sorting, and linear algebra.
Since the code is C, it can be utilized in C++ and C# programs as well. However, you can often find better implementations for the more common algorithms on CodeGuru etc. nowadays, already in your language of choice.
One used to be able download the code for free based on an "honor system" agreement to do so only if you already owned the book. Now you will have to buy a separate online subscription to do this, unless you have bought a recent (new) copy of the book which will contain a coupon.
The Numerical Recipes series is a real mixed bag. On the one hand, they present a mixture you can't find anywhere else: great descriptions of numerical methods alongside actual code. However, the code is encumbered with rather restrictive licensing, making it difficult to use in open source software.
For those comfortable with copyleft, the GNU Scientific Library may be a suitable alternative for the code aspect, but I'm still not sure what to recommend for those needing a more permissive license.
The book includes many algorithms. I like the texts for background introduction, but not the codes. The code part reads obsolete. It takes a long time for me to get used reading the code.