I've not seen a copy of Harmsworth in a leather binding - mine is in blue publisher's cloth - but if it is leather and you're likely to have further uses for the stuff I'd recommend Renapur Balsam:
https://renapur.com/product/renapur-...-balsam-200ml/
I'm about half way down a pot I've had for around 30 years, and was using it just this morning to refresh the half-leather bindings of seven volumes dating from 1831.
Paul