From the Heathrow airport website:

"Passengers will be able to purchase items from the departures lounge once they have passed through security and carry these onto the aircraft. However, if you are travelling to the USA, you will be able to purchase goods for consumption in the airport but no additional items, other than those on the list, can be carried onto the aircraft. These passengers may be subject to a secondary search at the departure gate before boarding. Any items not on the approved list will be removed."

http://tinyurl.com/f93tb

It sounds like you can buy a book once past security and bring it on board as long as you're not flying to the US.