This Small Cache Log Book from UKGeocachers is printed on Official RITR paper and has a plastic front and back cover.  It contains 30 single double sided lined sheets, and with the ring binder it not only makes the logbook easy to use, but you can also add extra pages if required.

The logbook measures approximately 7.5cm x 4.5cm plus an extra 1cm for the ringbinder.  Small enough to fit inside most small cache containers.