The only things that unite this movie with Forsyth's novel are the names of the characters and places. Eighty percent of the novel involves the setting up of a mercenary's mission to overthrow a dictator, with few instances of violence. This movie starts with a shelling of a runway where the mercenaries are escaping in an aircraft. Not so in the novel.
The whole 15 minutes of Walken's visit to Zangaro, meeting with a beautiful African woman and subsequent arrest with beatings and deportation are not in the novel.
The final assault in the capital is not accomplished by 11 men but by a whole group of howling mercenaries and loses much of the flavor of Forsyth's writing.
If you're looking for a close adaptation of Forsyth's novel, this isn't it. But if you like a good action movie with good characters, Walken et al provides this. I've read the book and I own the movie. I recommend both.