Let me rewrite the quotation using more standard styling:
You should never read just for 'enjoyment.' Read to make yourself smarter, less judgmental, and more apt to understand your friends’ insane behavior, or better yet, your own. Pick “hard books”: ones you have to concentrate on while reading, and for god’s sake, don’t let me ever hear you say, “I can’t read fiction. I only have time for the truth.” Fiction is the truth, fool! Ever hear of “literature”? That means fiction, too, stupid.
— John Waters, Role Models.
Hopefully, it is now clearer that "More apt to understand..." is the last item in a list.
Here is a list style guide I found that explains some related rules. Like most style guides, it makes the rules seem more strict than they actually are. Punctuation is largely an issue of style, and different authors will use it differently.
Horizontal vs Vertical Lists
When listing items inline, as in the quotation, it is called a horizontal list.
This is a vertical list:
This is a horizontal list: item, item, item, and item.
Lists of Sentences
When the items in a horizontal list are complete sentences, then it is okay to end each one with a period. This can be seen in Rule #9 of the linked style guide.
So, yes, it is sometimes okay to end each list item with a period. However, the list items in the quotation are not all complete sentences. They are mostly clauses, i.e. sentence fragments.
To be clear, let me reorganize the horizontal list from the quote as a vertical list.
Read to make yourself:
- Less judgmental.
- More apt to understand your friends' insane behavior, or better yet, your own.
As you can see, this is pretty non-standard. The first item on the list is the last clause of the sentence that introduces the list. The next item is a short adjectival clause. The last item is a long, complex adjectival phrase with it's own sub-phrases.
As I mentioned earlier, punctuation is largely an issue of style. This author has taken some liberties with their punctuation. I think the goal was to more clearly convey the cadence of the words when read aloud. Periods represent longer pauses than commas, and the exclamation point makes it clear that that particular item was said with enthusiasm. Using the periods to split list items also more clearly sets apart the last list item, which includes commas.
When I read this paragraph, I can hear the author speaking in a very assertive manner, full of hard stops.
So, strictly speaking, Mr. Waters' high school English teacher would probably mark that paragraph as incorrect. Luckily, works of literature are not edited by high school English teachers.
Punctuation in Quotation Marks
There are different opinions regarding punctuation in quotation marks. In general, you should end a quotation with the same punctuation you would use were it not a quotation. There are certainly exceptions, such as ending a sentence quotation with a comma when it appears in the middle of a sentence.
The author of the quotation has used a standard style for his quotation punctuation. "Pick hard books" is not a sentence, so you wouldn't normally end the quote with a period, but since he is ending each list item with a period, and this is a list item, it is ended with a period. This follows the guideline of using the same punctuation you would were it not a quote.
Beside which punctuation to use there is also the question of where to put it. Most schools in the US teach students to put the punctuation inside the quotation marks, as seen in this quotation. Some, such as myself, prefer the 'logical' or 'British' style where the punctuation goes outside the quotes.