There is a marvelous selection of duty free drinks available and sometimes there are special offers such as 2 for 1 or free gifts. If you are lucky you may be offered a free shot of whisky to settle your nerves before your flight!!!
Airport bookshops are good for finding reading matter to take when travelling. As I read a lot, and cannot find English books easily where I live, I try to get a stock when in UK, but they are heavy, so prefer to buy at the airport and then carry them on board . Some flights I have been on don't offer entertainment, so I find a book essential. Usually there are offers on 3 for the price of 2; or buy two and get the second half price.
What to buy: Usually modern writers or those in the best-seller lists but some classical writers can be found, as well as history, travel and light fiction.
What to pay: As in the high street, but also take advantage of special offers
W.H.Smith at terminal 4 has a good selection of books including bestsellers, fiction and non-fiction, travel guides, so why not pic one u for your flight. The shop is located in the duty free area.