What a wonderful analogy! Literature and fruit have many parallels (relationships) which can be derived from their similarities.
First, when one is looking at the surface of a text, they are losing the true "flavor" of what is underneath. In regards to the fruit, the skin of the fruit holds one thing and the core of the fruit holds another.
Second, one really needs to "sink their teeth" into a text to get the full meaning. In order to really experience a piece of fruit, one cannot simply hold it in their hand. Instead, they need to bite into it, taste it, devour it. In the same way, a text is only truly experience when a reader devours it.
One last way, specific to an apple, in which literature is like a fruit is the fact that apples come in many different varieties. Like literature, an apple speaks to a certain person based upon its appeal to the individual. Some people may prefer a green apple, over a red one, in the same way that they prefer tales of horror over poetry.