Two issues - saving changes in an instantiated workbook, and activating other workbooks - vba

Question

I have two spreadsheets; I'll call them spreadsheet 1 and spreadsheet 2. Spreadsheet one has a function which generates days of the month, and if it's at the end of the month, it is trying to call the module/sub in spreadsheet 2. This is to generate both "daily" reports and "monthly" reports.
At this point, there are two errors: the first is when I am trying to save the new instance of spreadsheet 2 that I created. The error is that it asks to save the workbook in a macro-free format. I simply want to save it! Not to make any changes to formatting. I am not even sure that it is trying to save changes to the instantiated book object.
the second is in spreadsheet 2, even though I set it to be active sheet (I think), the activesheet still comes up as the worksheet on spreadsheet 1 that runs the macro in the first place.
Any help is appreciated.
Option Explicit
Public Function LastWeekOfMonth() As Boolean
'finds the current date
Dim CurrentDate As Date
CurrentDate = CDate(ActiveSheet.Cells(FIRST_DATA_ROW, 1))
'find filepath and filename of the monthly documentation file
Dim mFilePath As String
Dim mFileName As String
mFilePath = "F:\Project Sweep\Kim Checklist\Barry Polinsky\Brathwaite, Tamika\"
mFileName = Cells(3, 4) & ".m_d.xlsm"
'if it is the last week of the month, write a monthly report, and return true to continue with the face to face paperwork
If (31 - Day(CurrentDate)) <= 7 Then
'write a monthly report
Dim app As New Excel.Application
Dim book As Excel.Workbook
' app.Visible = False 'Visible is False by default, so this isn't necessary
Set book = app.Workbooks.Add(mFilePath & mFileName)
'run the subroutine CheckSpreadsheet in module WriteReport in target book
app.Run "'" & mFilePath & mFileName & "'!" & "WriteReport" & ".CheckSpreadsheet", book
' CheckSpreadsheet (book)
'error next line
book.Save
book.Close
app.Quit
Set app = Nothing
LastWeekOfMonth = True
'if it is not, simply continue with the face to face paperwork
Else
LastWeekOfMonth = False
End If
End Function
In the target worksheet, in module WriteReport, subroutine CheckSpreadsheet, the following code is located.
Option Explicit
Public Sub CheckSpreadsheet(wbook As Excel.Workbook)
Set wosheet = wbook.Sheets("Monthly")
wosheet.Cells(5, 5) = "Hello world!"
End Sub

Answer 1

Don't need to have another instance of Excel, the property to hide a workbook is Windows, in order to hide the excel windows used by the workbook. Also bear in mind that a workbook can have more than one window.
If you are sure that the workbook you want to hide has only one window use this line:
Workbooks("WbkName").Windows(1).Visible = False
If the workbook has several windows use this procedure:
Sub Wbk_Hide()
Dim wbk As Workbook, wdw As Window
Set wbk = Workbooks("WbkName") 'Update as required
For Each wdw In wbk.Windows
wdw.Visible = False
Next
End Sub
I believe this changes the scope of your procedures, let me know otherwise.