Hellblazer is a Vertigo comic book starring John Constantine of Swamp Thing fame. The series follows the trenchcoat wearing magus during his adventures through the underbelly of a dark and mystical world. The series can vary from very upbeat (Constantine versus demon yuppies) to very dark (an early story line about a fly demon). The series has been running since the 1980's and is still going strong today. Many of Vertigo's good writers have done issues of Hellblazer (such as Garth Ennis' vampire king storyline), and I believe the covers are always illustrated by Glen Fabry (who also did the covers for Vertigo's Preacher. Vertigo has released a number of TPBs starting with Original Sins and eventually releasing at least four others.