I own the book but I haven't read it yet. With Stephen King you can usually expect at least a little bit of language, but his graphicness and violence level depends on the book in question (and really, he's never TOO bad that way, his horror is typically more about the horrors of the human mind and/or the mind's ability to withstand horror, more than about the graphic violence or the detailed descriptions of gore; that said, he definitely doesn't shy away from describing bodily wounds).

It's my understanding that the movie was a fairly faithful representation of the book. At a guess I'd say that you'd get a few nasty descriptions of things, for example what a person smells like when they're electrocuted, but otherwise it should be similar to the movie.

Frank Darabont (the script writer and director) does very good representations of King's work. In addition to The Green Mile, he also wrote scripts for and directed The Shawshank Redemption and The Mist, both also originally by King.

Conclusion: I can't promise you'd enjoy every Stephen King novel, but The Green Mile is probably relatively GFT.