When people ask me what the story is about, in typical former film student fashion, I fall back on the stereotypical Hollywood "This story is X meets Y" method of explaining a plot, so I tell people "The book is Bronson's Death Wish meets his movie The Mechanic". One part vigilante crusade, one part contract killer origin story.
With that in mind, I provide trailers to both films. The second trailer is newer than the theatrical release; I'm guessing it was included in a DVD of some sort. It does, however, show the dynamic between Bronson and Jan Michael Vincent's characters a lot better.
If you've never seen either of these movies, I urge you to check them out - both are great 70's action thrillers.