Just learned about this handy tip for Apache:
RewriteEngine on
RewriteRule ^/book/([^/]*)/([^/]*) /cgi-bin/book.cgi?author=$1&subject=$2 [PT]
It rewrites a URL so it can be arguments to a cgi program so something like
can be parsed as
http://server/someDirectory/someCGI.cgi?path=blah
Handy!