The following is a list of items considered to be canon in the Babylon 5 universe.

Dell Novels

Dell Book #7 The Shadow Within by Jeanne Cavelos
Dell Book #9 To Dream in the City of Sorrows by Kathryn M. Drennan

Del Rey novels

Dark Genesis: The Birth of the Psi Corps by J. Gregory Keyes
Deadly Relations: Bester Ascendant by J. Gregory Keyes
Final Reckoning: The Fate of Bester by J. Gregory Keyes

Legions of Fire Book I: The Long Night of Centauri Prime by Peter David
Legions of Fire Book II: Armies of Light and Dark by Peter David
Legions of Fire Book III: Out of the Darkness by Peter David

The Passing of the Techno-mages Book I: Casting Shadows by Jeanne Cavelos
The Passing of the Techno-mages Book II: Summoning Light by Jeanne Cavelos
The Passing of the Techno-mages Book III: Invoking Darkness by Jeanne Cavelos

Short Stories

"Shadow of His Thoughts" by J. Michael Straczynski (Amazing Stories #597, Summer 1999)
"Genius Loci" by J. Michael Straczynski (Amazing Stories #599, Winter 2000)
"Space, Time, and the Incurable Romantic" by J. Michael Straczynski (Amazing Stories #602, Summer 2000)
"Hidden Agendas" by J. Michael Straczynski (Official Babylon 5 Magazine #22, May 2000)
"True Seeker" By Fiona Avery (Official Babylon 5 Magazine #23, Jul 2000)
"The Nautilus Coil" by J. Gregory Keyes (Official Babylon 5 Magazine #24, August 2000)

Comics

Issue #1 "In Darkness, Find Me"
Issue #2 "Treason"
Issue #3 "In Harm's Way"
Issue #4 "The Price of Peace"
Issue #5 "With Friends Like These..."
Issue #6 "Against the Odds"
Issue #7 "Survival the Hard Way"
Issue #8 "Silent Enemies"
Issue #9 "Duet for Human and Narn in C Sharp"
Issue #10 "Coda for Human and Narn in B Flat"
Issue #11 "The Psi Corps and You"

Issue #1 "In Valen's Name"
Issue #2 "In Valen's Name"
Issue #3 "In Valen's Name"

Lost Tales/SDCC Comic

Discussion

The following quotes are provided of Straczynski discussing the above items:

In 1995 on the moderated group, Straczynski gave a broad approval as to the canonicity of the original Dell novels and the DC comics: "The books are canon in broad terms, in that the events generally did happen, but the details can vary given that you've got diverse hands doing the work (including the comics)."

In 1997 on the moderated group, Straczynski spoke in more detail about the six original Dell novels and the three final Dell novels: "In the past, we've taken pitches, but overall I've felt that the books strayed too far from canon by doing it that way; the latest batch were more directly influenced. One is taken right out of B5 canon, the Anna/Icarus story, and it's one of the best to date; the second one up is based on a premise I wrote up and was assigned to the writer (it's the weakest of the three, set on Centauri Prime while G'Kar was still held prisoner, but still okay); the third is also based on a premise, and follows Sinclair after leaving B5 to form the Rangers...I worked very closely with the writer here to ensure that virtually *every line* is canon."

Of The Shadow Within, Straczynski stated in 1997 on CompuServe that the "story of the Icarus as presented in this book is considered canon."

Of To Dream in the City of Sorrows, Straczynski stated in 1997 on the moderated group that "of all the novels, this one is 100% canon...it's also the most exhaustive attempt to tie in all the storylines done to date. Anybody wants to know what happened to Sinclair, it's in the book. That's a genuine chapter of the story arc."

In 1999 on CompuServe, Straczynski spoke of the Del Rey trilogies (the Centauri trilogy, the Techno-mages trilogy and the Psi Corps trilogy) as well as Cavelos' The Shadow Within and Drennan's To Dream in the City of Sorrows, both of which were republished by Del Rey: "the new novels from Del Rey that I'm writing the outlines for are canonical, the Drennan novel is nearly all canonical, much of the Cavelos novel is canon."

In 2000 on the moderated group, Straczynski spoke to the degree of canonicity in the Del Rey trilogies, which he wrote the outlines for. As a criteria for whether something can be considered canon, Straczynski notes that it "tends to be something that I generate, as with the B5 trilogies from Del Rey. They are canonical in the broad strokes, though it's impossible to ride every single small detail and get anything done on any kind of reasonable schedule."

Straczynski wrote the first Babylon 5 comic, and noted, in 1994 on the moderated group, that he'd "supplied the premise" for the remainder of the first four-issue arc. In 1996, Straczynski posted the following on CompuServe:

I will not, however, be writing directly for the comic anymore. DC has a policy that they will not allow writers final say on their books. Only the editors. On all books written by other writers, as per their contract with DC, I via WB, have final approval. However, if I function as a writer, then I must give up that right and allow myself to be rewritten by the editor. It's a jurisdictional matter. Suffice to say I was appalled by some of the rewriting done on what I'd turned in on the four issue arc.

Consequently, I will not be writing that arc, or any other books for the DC title. I will be happy to provide information to keep the story current with the series, farm out premises so it stays canon, and work closely with the writers in my approval status. That way I can make sure the work is to our standards; if I write for the book, I cannot do so, because the editor insists on rewriting me to maintain DC's jurisdictional structure.
Of the remaining comics, Straczynski was credited for the story premise of issues 5 through 8, issues 9 and 10 were written by David Gerrold, and the eleventh and final issue was a Psi Corps propaganda booklet, of which Straczynski noted: "Only about 10% of what's recorded in that booklet is true, and even that's distorted. The character in the booklet is totally fictional." Gerrold's two-parter, originally pitched as a Babylon 5 episode, was recommended to DC by Straczynski, according to Gerrold's comments in Babylon 5 Scripts: Other Voices, Volume 1.

Of In Valen's Name, Straczynski posted the following on the moderated group in 1997: "It'll first appear in the UK, serialized in the B5 magazine (which will be regularly published over there shortly, to coincide with new episodes), then it will be gathered together and published in the US by DC Comics. The books are being written/assembled in-house, to avoid conflicts with editors. I've written the first issue, which brings the Valen story into the foreground, and the second two parts are being written by Peter David."