Okay, so I'll spoil it for you. Digory does not take the apple to his mother as he was tempted to do by Jadis. He does what he should, for the sake of Narnia, and takes the apple back to Aslan.
Aslan has him throw that apple, and, of course, like all things in Narnia that day, it fell into the ground, and a tree grew from it almost immediately. Guess what...it was an apple tree. Aslan does allow Digory to take one of these apples to his mother, though. The point is that a stolen apple from the garden would have done what it was designed to do...heal Digory's mother...but would have brought sorrow along with it. Now, with an apple that Digory was told to take, it will bring joy.
The fruit of the Tree of Knowledge in the Garden of Eden was not itself bad. It worked exactly as it was created to...giving knowledge of good and evil. The fact that it was taken when it was not allowed is what made it a sin. But just as in Narnia, another tree became the salvation of Narnia (for a time), there is another tree--The Tree of Life--which we will one day be allowed to eat of, and then, what rejoicing there will be, for we will eat of this tree in the proper time.