It's the V5 document you need with your registration details on.
Never quite understood why it's called a logbook when it's not a book.
It used to be. It was a green fold up book that had details entered manually.
[url width=800 height=602]http://volvo1800pictures.com/a_S_67_1/67_79_id_1087_green_logbook_1.jpg[/url]
I've still got the original one for my old Matchless motorbike.