[Gauthier]

Using Word 2010 64bit Edition
May I suggest the following changes:

In vbaKernelStuff

#If VBA7 Then
Private Declare PtrSafe Function WaitForSingleObject Lib "kernel32" ( _
    ByVal hHandle As LongPtr, _
    ByVal dwMilliseconds As Long) As Long

Private Declare PtrSafe Function OpenProcess Lib "kernel32.dll" ( _
    ByVal dwDesiredAccess As Long, _
    ByVal bInheritHandle As LongPtr, _
    ByVal dwProcessId As LongPtr) As LongPtr

Private Declare PtrSafe Function CloseHandle Lib "kernel32" ( _
    ByVal hObject As LongPtr) As Long
#Else
Private Declare Function WaitForSingleObject Lib "kernel32" ( _
    ByVal hHandle As Long, _
    ByVal dwMilliseconds As Long) As Long

Private Declare Function OpenProcess Lib "kernel32.dll" ( _
    ByVal dwDesiredAccess As Long, _
    ByVal bInheritHandle As Long, _
    ByVal dwProcessId As Long) As Long

Private Declare Function CloseHandle Lib "kernel32" ( _
    ByVal hObject As Long) As Long
#End If

#If VBA7 Then
Dim TaskID As LongPtr
Dim ProcHandle As LongPtr
#Else
Dim TaskID As Long
Dim ProcHandle As Long
#End If

Without the above, it won't compile.

In datBookCreatorProperties, Sub INITIALIZE()

#If Win64 Then
        setCalibrePath "C:\Program Files (x86)\calibre"
#Else
        setCalibrePath "C:\Program Files\calibre"
#End If

That correction will only correct the path for 64bit word
Not sure how to correct it for 32bit word on Win64.

In eBookCreatorCalibre Sub html2any

Adding a bit of diagnostic on the calibre path, otherwise, it fails silently

' Prepare to create the eBook
    szHTML2ANY = Replace(szPathCalibre, """", "") + "\ebook-convert.exe"
    If Not IsFileExists(szHTML2ANY) Then
        MsgBox "Calibre Path Incorrect"
        Exit Sub
    End If
    szHTML2ANY = """" + szHTML2ANY + """"
    AnyToHMTLToTarget szHTML2ANY, szFilepath, szFilename, szParam, True, eEBookFormat

Remark, I only corrected the NOIMP version, there may be more.
Also the BrowserFileSystem need a bit of improvement.