Back in the old days, I had a book that listed many things about math tables and symbols. What is the modern equivalent book?
My question was unclear. I think it was because I used one question in the title and another in the body of the post. Leave it closed.
This symbol has no "modern equivalent," to my understanding, because it is still in use today.
$x \mapsto y$ means, for an implied function $f$, that $f$ maps $x$ to $y$, i.e. $f(x)=y$. You might have each side of the arrow written in a general form, such as $x \mapsto x^2$ denoting $f(x)=x^2$, or to denote a particular mapping of elements, such as $2 \mapsto 4$ as under the previous example.