I've been searching since a few days the symbol of Napier's number (e).
It is a symbol of and with a cut on the inside similary to the double struck commands (
\mathbb). I vaguely remember that there is a package, perhaps distributed by Wolfram Alfa in a
.zip file, which it can to be used in LaTeX documents. Currently I have not been able to find it.
Here there is a ugly screenshot (edit: slightly better screenshot from Wolfram|Alpha):