Oh, my goodness. This is the silliest question ever. The new Common Core Standards believe that 70% of what students read in high school should be non-fiction, and this has many people up in arms about this proclamation, claiming that this will kill Shakespeare (again) and bore children to tears with manuals and memos as their primary texts.
So what's the real answer?
Everything. They should read everything. Street signs, bumper stickers, cereal boxes, trashy magazines, literature, the newspaper, internet memes, comics, manga, sports programs, subtitles, credits, captions, manuals, emails, text messages...
You name it, they should read it.
When kids are learning how to read, the ones who are exposed to a wider variety and frequency of environmental print do better; they learn more quickly, and they understand better the functions and sounds of all of those lines, squiggles and dots. As kids move from learning to read to reading to learn (in and around the 3rd or 4th grade, in the absence of any learning issues or exceptionalities), this foundation with language and familiarity with all of its uses helps them to understand what they read more.
Essays, editorials, letters, poetry, reports, primary source documents, biographies, summaries, personal narratives...
Everything. They should read everything. In middle school, the hardest age to get most kids to read, the focus should be on letting them read whatever they want for pleasure, requiring them to sit down and read for a set period a day, every day, whatever they want, gradually introducing other forms in class (or whatever schoolish configuration you use) and connecting them as much as possible to their world and the reading they enjoy. It's like fishing. Don't bait the hook with a brussel sprout. A fish wants a fat, juicy worm. Let them read a zombie story if they want, like World War Z (a personal favorite), then teach geography with a non-fiction text, using the book (document the spread of zombies across the globe. I am telling you: you MUST read this book. It's like non-fiction. But not. You could teach oral histories with this, too, and epidemiology. I digress).
EVERYTHING. Did I mention you should let kids read EVERYTHING?