double \\ are needed internally to pass a \ as part of the string (and for the Log. do not use for the command)
it is called Escaping. \ is a escape character. to Not have it be an Escape, you need to 'Escape the next character. thus \\
Code:
C:\\Files\\Calibre-Books'
is NOT a program (it may be a Library)
the (64 bit version) Program is C:\Program Files\Calibre2\calibre.exe