In the past, the Numerical Recipes book was considered the gold standard reference for numerical algorithms. The earliest Fortran Edition was followed by editions in C and C++ and others, bringing it then more up-to-date. Through these, it provided reference code for the state-of-the-art algorithms of the day.
Older editions are available online for free nowadays.
Unfortunately, I think it is now mostly useful only as a historic tome. The "software engineering" practices seem to me to be outdated, and the actual content hasn't kept pace with the literature.
What similarly comprehensive yet approachable references should the modern programmer look at instead?