Button Man ran for four series, and remains one of the best-selling collections of 2000 AD strips (other than Judge Dredd) ever published.
The series outings, all written by John Wagner, are:
Button Man:
"Book I: The Killing Game" (with Arthur Ranson, in 2000 AD #780-791, 1992)
"Book II: The Confession of Harry Exton" (with Arthur Ranson, in 2000 AD #904-919, 1994)
"Book III: Killer Killer" (with Arthur Ranson, in 2000 AD prog 2001 & #1223-1233, 2001)
"Book IV: The Hitman's Daughter" (with Frazer Irving, in 2000 AD #1551-1566, 2007)
http://en.wikipedia.org/wiki/Button_Man