1. See the film first, and then read the book. If you feel like it.
Films are almost never as good as the books they're adapted from, that's an unwritten rule of the Universe. First things first, think about how long a film is: at most 3 hours. How could you possibly squeeze, say, 1000 pages of literature into 3 hours on the big screen? You'd either have to be a God of script writing, or leave out a lot of events/details.
Take into account the fact that the author of the book only works on the script in few cases. As everyone has different ways of envisioning something, the script will most probably end up changed from the original material. In some cases, drastically. If you watch the film first, you might discover the book is much better (or the other way around).
As a general rule, try to keep your expectations minimum when you read a book after you’ve seen its film (or vice-versa). Keep an open mind and you most likely won’t be disappointed in the end.
Furthermore, some matters addressed in books are controversial even to this date, and cannot be handled lightly or shown on screen without certain risks (like upping the rating, therefore losing profit, or creating mass disapproval). Let's say there's a graphic rape scene in a book, and it's essential to the plot - to show this, a director would have to have guts and a great deal of talent (anyone can make tasteless films, it's transforming the unpleasant scenes into something artistic that's important). So, don't complain if certain scenes are left out or modified; you might have heard of this scene and want to see it, but the director doesn't want to film it.
Remember, your imagination is more powerful than whatever you see on-screen – a monster created by your mind may be more frightening to you than what a director shows you. That’s why the best films/books are the ones that don’t give out many details: so the reader can imagine what they fear most. Consequently, this makes said works appeal to a wider audience/public.
Example #1: I saw Harry Potter 5 with my best friend, who has seen the rest of the films, but hasn't read the books. Throughout it, she kept asking me why what was on screen was happening, proof of the fact that key details were left out (since I can hardly consider my best friend an idiot, objectively speaking). As for me, I liked Harry in the books, but found his character one-dimensional and annoying in the films.
Example #2: I find Stephen King's books brilliant. The films, however, are a different story. I tried to watch IT two times, and turned it off both. There was just too much left out, the pacing was off in some parts and further explanation was needed, and explaining the missing parts to someone else would be annoying because I’d have to pause it constantly.
Bottom line is, if you don't read the books, you may miss a lot of details that contribute to the storyline and character development. Some characters don't translate well to screen or the time constraint makes them lack depth, so having a complete picture is preferable to a disjointed one.
