On Linux:

$ man execve

From the DESCRIPTION section, first paragraph (RedHat 6.2 anyway):

   execve() executes the  program  pointed  to  by
   filename. filename must be either a binary
   executable, or a script starting with a line of
   the form "#!  interpreter  {arg}". In  the latter
   case, the interpreter must be a valid path­ name
   for an executable which is not itself a script,
   which will be invoked as
   interpreter {arg} filename.

So yeah, if you know where to look or dig around enough, it's there to be found without buying a book. Such is often the case with Perl/perl, Linux, BSD, Postfix, Apache, and many other projects in the OS/FS world.