.Hack//AI Buster Vol 1 & 2 (novels)
.Hack//Sign (Anime) (only Eps 1-27)
.Hack (first 4 games Infection, Mutation, Outbreak, and Quarantine) & their Liminality DVDs
- The last .Hack//Sign Anime episode 28 (Unison) actually takes place here in the timeline after the last game
.Hack: Legend of the Twilight Vol 1-3 (Manga)*
.Hack//GU (Newest games)
There is also a .Hack//ZERO novel which appears to exist between //Sign and the first game, but your not reading it unless you read japanese at the current moment.
Also some small manga comics have recently come out based on the current game. I dont think any of the comics are cannon except maybe .Hack GnU. And thats a maybe. They are also in japanese still unless you can find a fan translated copy. Edit: Oops, also forgot about the recent //Alcor manga and //CELL novel which are probrobly cannon. All three are serialized in "The World" magazine and only in japanese at the moment Edit2: Alcor is most likely not cannon, but I wouldnt be too sure to completely disclaim it yet.
Also theres .Hack//Another Birth novels which is the first games told from BlackRose point of view. Don't know how cannon they though.
*There is a Legend of the Twilight Anime, but it is considered non-cannon with a different storyline.