[Gauthier]

The calibre path saved by setCalibrePath
is machine dependent and not really a document Property thus use:
and of course also do the verification there

Sub setCalibrePath(szPath As String)
    SaveSetting "BookCreator", "Calibre", "CalibrePath", szPath
    If Not IsFileExists(Replace(szPath , """", "") + "\ebook-convert.exe") Then
        MsgBox "Calibre ebook-converter Not Found there"
    End If
End Sub

Function getCalibrePath() As String
    getCalibrePath = GetSetting("BookCreator", "Calibre", "CalibrePath")
End Function

That will save to the registry.