Dear Ken:

Made a mistake; I used the wrong MDB. Here is the correct AfterUpdate code,
which still seems unrelated to the problem to me...:

Private Sub cboAuthor_AfterUpdate()
On Error GoTo Errorhandler
Dim db As Database, ssql As String

ssql = "INSERT INTO Tbl_BookAuthor (Book_ID, Author_ID) SELECT " &
Forms!Frm_LibraryDataEdit.txtBook_ID & " AS Expr1," &
Forms!Frm_LibraryDataEdit.cboAuthor & " AS Expr2"
Set db = CurrentDb()
db.Execute ssql, dbFailOnError

If db.RecordsAffected <> 1 Then
   MsgBox "This is not a valid Book ID. Please try again.", vbOKOnly +
vbInformation, "W. Ross Macdonald School"
   Me.Undo
End If

   Set db = Nothing
   Me.Refresh
   Forms!Frm_LibraryDataEdit!cboAuthor = Null
   Forms!Frm_LibraryDataEdit!cboAuthor.SetFocus

ExitPoint:
   Exit Sub
Errorhandler:
   If Err.Number = 3134 Then
   MsgBox "You must enter the title information first.", vbOKOnly +
vbInformation, "W. Ross Macdonald School"
   Me.Undo
   Me.cboAuthor = Null

   Else
       fncWRMSErrMsg Err.Number, Err.Description
       Resume ExitPoint
   End If

End

<snip>

Hmmmm. The workaround of resetting the focus is not intended to be used for
a NotInList situation, but rather for closing a combo box list that was
opened via other means (such as .DropDown method).

I set up a small test form where I put a combo box (limit to list = yes;
event procedure for not in list event as noted below) and a text box on the
form.

Private Sub Combo2_NotInList(NewData As String, Response As Integer)
MsgBox "no"
Response = acDataErrContinue
Me.Text0.SetFocus
Me.Combo2.SetFocus
End Sub

When I open the form and type in a value that is not in the combo box's list
and hit tab, I get into a continuous loop in the NotInList procedure. I note
this to you because your code has similar construction for when the person
does not want to add the new value. To avoid this loop, the code has to
include an Undo step:

Private Sub Combo2_NotInList(NewData As String, Response As Integer)
MsgBox "no"
Response = acDataErrContinue
Me.Combo2.Undo
Me.Text0.SetFocus
Me.Combo2.SetFocus
End Sub

You should modify your current code accordingly?

Now, to the dropdown issue -- the dropdown list shows because of the
NotInList event occurring. (You can see this if you add a loop into the code
(after setting focus to the other control) to DoEvents up to 500 times --  
when the focus goes back to the combo box, the list drops down.) I don't
know of any way to turn this "feature" off.

After I read your reply, a faint memory tickle came into my head that I'd
had a similar situation in a database and had programmed around it.. went
looking for it, but couldn't find it.

So, I put my thinking cap on and came up with a "small" workaround. Turn off
the LimitToList property and remove your code from the NotInList event.
Instead, I think you can use the BeforeUpdate event of the combo box to test
for whether the item is in the list or not, and then to open your popup
form:

Private Sub Combo2_BeforeUpdate(Cancel As Integer)
Dim varValue As Variant
If DCount("*", Me.Combo2.RowSource, "FieldName=" & Me.Combo2.Value) = 0 Then
   Me.Combo2.Undo
   Cancel = True
   If vbYes = MsgBox("add the item?") Then
       DoCmd.OpenForm "Frm_InsertAuthor", OpenArgs:=NewData,
DataMode:=acFormAdd, Windowmode:=acDialog
' rest of your code to test for whether item was inserted or not goes here
' including the next requery step as appropriate
       Me.Combo2.Requery
   End If
End If
End Sub

A bit of a workaround, yes.....