But it can and does attract import VAT:

All other goods If you order or send purchased goods other than alcohol, tobacco, perfume and toilet water from a country outside the EU then you: don't have to pay Excise Duty may have to pay Customs Duty on goods with a value that exceeds £135 will have to pay import VAT on goods with a value that exceeds £15 Note that on all goods from outside the EU, Customs Duty is waived if the amount of duty calculated is £9 or under.

Source

My problem is, as the recipient of a package I never asked for and never agreed to receive, with no signature from me at any point, I was held liable for a not-inconsiderable sum of money.

EDIT: Also, according to the same source as above, the following holds:

It must have been sent from a private person outside the EU to a private person(s) in the UK.

and

There must be no commercial or trade element and it must not have been paid for by the recipient either directly or indirectly

So a complimentary copy of a textbook sent by a publisher would not count as a gift.