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As with much of modern mathematics, the symbol "e" was inroduced by Leonhard
Euler as "that number whose hyperbolic logarithm = 1" begining in a manuscript
written probably in 1727 upon his arrival in Russia.  He used it numerous times
subsequently, and it appeared in print for the first time in Euler's
_Mechanica_ in 1736.  Euler is also largely responsible for the current use
of the symbol for pi, and he introduced the symbol "i" for SQR(-1).  All of
these, plus 0 and 1 can be combined in the equality:
e^(pi*i) + 1 = 0

See _A history of mathematics (2nd edition)_ by Carl B. Boyer, page 442.

John R. Vokey
Department of Psychology
University of Lethbridge
Lethbridge, Alberta
CANADA  T1K 3M4
vokey@hg.uleth.ca

p.s.: although not known for sure, it has been suggested that "e" was chosen
for "exponent".  Given that he wrote mostly in Latin (and also French), his
native language was German, and he was trained in oriental languages, spoke
and wrote fluent Russian, and had little difficulty with _any_ European
language as he moved from country to country, who knows why he chose the letter
"e".