As I understand it, there is no import duty on books into Britain. As a caveat, it was a few months ago I looked and the relevant website (which sadly I can't remember but I would advise satisfy yourselves anyway) implied 'books for reading'... are there others?

Apart from which, if the package has a value of under 18 GB pounds it would probably not be considered anyway.

The real pain is the charges applied by Royal Mail (or other parcel deliveres e.g. Fedex) if they have to collect any duty.