You Should Already be Reading: ‘Hellblazer’
Hellblazer, if you don't already know, is a series about a man named John Constantine. He is a heavy drinking, chain smoking magus with an attitude who will charge into Hell with a squirt-gun, or side with Hell ... whichever serves his interests better. In the comics upon which the movie was based, however, Constantine looks like Sting, not Keanu Reeves, and he is British.
John Constatine can be described as a man that will do almost anything to advance his own interests or save his own skin, including being a party to the deaths of his friends. You cannot really call him a good guy, but you have to give him some credit for trying to be good every once in a while. It's hard staying alive using only your wits (no holy shotguns here) and magic with representatives of both Heaven and Hell coming after you.
The best way to get to know Constantine is to read the collected books. If you want a little dose of some Hellblazer you can read the first issue on the Vertigo site where the first taste is free. As for which books you should read first, my recommendation would be to just grab whichever ones your local shop has in stock and read them immediately. Most of the storylines are pretty independent of each other, so if you happen to read them out of order there is no real harm done (in my opinion).
The character was created by Alan Moore with artists Steve Bissette and John Totleben during their now-legendary run on Swamp Thing. When Constantine got his own series, Jamie Delano and artist John Ridgway were the original creative team on the Hellblazer title. Since then, there series has been carried on by a large number of writers and artists. Hellblazer writers have included (but are not limited to) Garth Ennis (my personal favorite), Warren Ellis, Brian Azzarello, Mike Carey, Grant Morrison, Denise Mina, and currently Andy Diggle. There was even one issue by Neil Gaiman and Dave McKean. A few of the artists who have graced the book are Steve Dillon (on the entire Garth Ennis run), Marcelo Frusin, and currently Leonardo Manco.
Finally, if you are a Hellblazer fan, you should have already picked up #233. It finishes the two-issue story of "Wheels of Chance, Systems of Control," which has Constantine confronting his past and his memories of Newcastle and Ravenscar. Even though it is a short bit of a story, it sets the plot up for some really big encounters to come. So, if you haven't read Hellblazer before or have strayed from the path, now is a good time to find your way home.