Like the dumb kid in biology class, movies are only as good as the people they copy off of. When Hollywood isn't busy cranking out sequels, it's scouring Earth for stories and characters that will save them the hassle of coming up with their own.
In fact, you can make a pretty good guess as to the quality of a film just by finding out what it's based on. We've helpfully ranked these sources from best to worst in the order of how likely it is the resulting film will suck.
The Case For:
Since the first narrative film, The Great Train Robbery was based on an 1896 story by Scott Marble, cinema has been plundering the written arts for ideas. Literature is great because you've got two built-in audiences right from the start: people who liked the book and people who think they may like it, but won't take on the onerous burden of actually reading it.
Plus, it' a lot easier to work with an author or a playwright than a screenwriter, because most of them are either irrelevant old fogies who don't mind getting their work "reimagineered," or better yet, dead.
Cases in Point:
Jaws, The Godfather, Goodfellas, The Bourne Ultimatum, The Silence of the Lambs, A Street Car Named Desire, Lawrence of Arabia
The Case Against:
Hollywood producers seem to base most of their book adaptation choices on the sales of the book, as it's considered unreasonable for them to actually read all 300+ pages of one. That's dangerous, because not every book translates to the screen. If it involves lots of fucking, people chasing each other through stairwells or both at the same time (John Grisham, Michael Crichton, Charles Dickens) then you're in the clear. If most of the action occurs in the characters' head (Albert Camus, James Joyce, Virginia Woolf), you're going to end up with an $80 million staring contest.
Cases in Point:
All The King's Men, Fast Food Nation, Divine Secrets of the Ya-Ya Sisterhood, Everything Is Illuminated, The Namesake
The Wild Card:
Not that all books have to be War and Peace, but it' somewhat surprising to find out a movie like Death Wish is based on a book. It's a book that we've never read, but feel confident assuming contains the following sentence: "He punched the scumbag with his big, ham-like fist, and that's when he noticed something behind him explode."
Lord of the Flies + Lolita + Fountainhead + Huckleberry Finn + 1984 + Jane Eyre = "Movies you can watch instead of studying for your high school English exams."