Cut the pie in half with one vertical cut through the centre (that is, how you would normally cut a cake in two). Then cut it into four with a further vertical cut through the centre, at right angles to the first (again, all normal so far). Then stack two pieces on top of the other two, and make one further vertical cut through all four pieces, making eight pieces.
Some people offered a slightly different solution, cutting the pie horizontally after cutting it in four. I don't think I'd be pleased to get a piece of apple pie with no crunchy piecrust on top. But I didn't specify "eight equally appetizing pieces" so I accepted this solution.
There were seven correct answers in all, and the winner (pulled out of a hat this morning by Eleanor) was Matt Shacklady of St. Helens. Congratulations, Matt. I gave the books to your mum and sister as as I passed through St Helens today. They seemed really nice. Say "hi" to them for me.