It's really a question of preference. Do you read the book, first? Do you attend the movie and then read the book? Or do you do the "one or the other but not both" thing.
There is a reason to ask. Of course.
I am currently reading several books at once. It's not something I do frequently, especially considering how much time reading can consume and how little time I actually have to myself. But, at the moment, I am.
Most of my reading material now lives on one of two Kindles, and another book is being read on an iPad Mini. My problem, however, made itself manifest when I stopped at a new Goodwill store just north of town, to look around.
I found a copy of The Shipping News and paid 89 cents for it, took it home, and sat down with a "cup of coffee and a good book". Quoyle (the protagonist), as described in the book, looks nothing like Kevin Spacey (protagonist - movie), at all. And a big element of Quoyle's personality arose from his lifelong embarrassment over his big chin. The book's description makes him look more like a Tonight Show host than Kevin Spacey.
So that's one thing.
Then there's the difference in setting. The movie version of Newfoundland and the book version look nothing alike. Cliffs in the movie, barren rocky shores in the book.
That's another thing.
In the book, Quoyle has two daughters. In the movie? One.
I love the book, and now I don't like the movie as much. It's like I was lied to.
I think I'm going to make a "sort of" rule for myself: If it's a book - read it, don't "see" it. If it's a screenplay, see it - and maybe read it when the book comes out. And if you must see it after the book has been read, learn how to compartmentalize.
Keep your sanity - because the difference between book and movie WILL irritate the crap out of you.