While Numerical Recipes is quite useful, I would hardly call it a "national treasure." Numerical Recipes gets recommended a lot because it is one of the more comprehensive references available on this subject but you should know that it's not perfect and has many flaws. The 'C' code in it is particularly horrendous. The Fortran version is somewhat better. The C code in Numerical Recipes reminds me of an old "mainframe hacker" who once told me "I can write Fortran in any language..."

There's a page at NASA's Jet Propulsion Lab that's titled "Why not use Numerical Recipes." It's worth reading before jumping head first into the book and includes suggestions for alternatives to Numerical Recipes.

That said, Numerical Recipes is certainly a useful starting place for numerical algorithms but remember that TMTOWTDI and this book only shows one way.