The first thing you need to do is determine the data type of your column and also it is helpful to know the name you assigned the column. The following would work with a data type of Yes/No
Code:
If Me.ControlName.Value = True Then
MsgBox "This book is not avaialble."
ElseIf Me.ControlName.Value = False Then
'TODO Create logic to process the
'book being checked out
Else
'TODO Create Error Trapping
End If
The above code depends on a control that is bound to the field in your table. Aslo, you may be able to get away without the ElseIf.
Code:
If Me.ControlName.Value = True Then
MsgBox "This book is not avaialble."
Else
'TODO Create logic to process the
'book being checked out
End If