Canon...

What do you consider canon for the Dark Eldar?

The Codex is the most obvious go to place for all background and fluff as well as the few pieces that appeared in White Dwarf and the site (colours of the Kabals articles) afterwards.

Now that Path Of The Renegade is about to be released do you consider its contents 'official'?

How about the unretconned elements of the previous codex and other appearances in BL books and elsewhere?

Is it Black Libary? The Path of the Renegade I mean. Because if it is then it's very canon.

White Dwarf is about as canon as the Black Libary, but harder to vertify.

For the unretconned pieces, well they are likely retconned if something in the newer codex contradicts it, if not they are likely good to go.

I see it like this:

Codex: canon
Unretconned parts of old Codices: canon
White Dwarf: canon

Black Library novels: not strictly canon, but may make their way into the next Codex and become canon.
So, I may use the term semi-canon here.

This is just the way I do things; there have been too many terribly out of place things in some of the novels for me to be able to treat them all as canon, but they do vary,some are much better for that and maybe Path of the Renegade will be one of these Again I must stress that this is just what I think, it's the last thing I remember hearing from GW but I know this can be a heated subject - I no longer visit a forum where I got attacked for daring to say that novels are anything less than 100% canon 0_o

But for the purposes of roleplaying only rather than fluff discussion, if a bit of background fits with the theme then I use it.

Oh and: Dark Heresy, Rogue Trader etc roleplaying game books - looks pretty canon so far as I can tell?

40k in general has very loose canon. Everything is canon, but since a lot of it is written by different people, with sometimes conflicting views and their own ideas on how things work, you end up with a lot of contradictions and pretty much have to choose what part you consider canon and what not. Codices and other official game fluff trumps BL novels and third-party fluff. I wouldn't consider every story where a single Spesh Muhreen defeats a billion Orks/Daemons/Tyranids/whatever with only his combat knife canon (GW themselves have mentioned in some White Dwarf article that that's not really an accurate representation of Marines, and and things often get exaggerated to make for a more dramatic story). Most BL novels are more like fanfiction somebody is paid to write. I usually only consider BL books canon if they elaborate on the existing background and bring some new light to the setting. FFG books would technically be less canon that even BL novels since they're third party, but their fluff tends to be much better written.

I'm with Lady Malys on this one. Too many instances in BL stuff that doesn't seem to fit with the codex background, like Chaos Marines in Razorbacks in DoW, and one that really bugged me was the references to numbers of marines in the Deus Blood Angel books, he seemed to be throwing them away in IG like quantities.

That being said, I think the effectiveness of Marines is dumbed down from GW's idea in game terms so that a space marine army for a 2000 point game doesn't consist of 5 guys.

Ok so I've digressed a bit, I'd go with codices as canon (new primarily and old where it's not contradicted later), White Dward as canon, and BL stuff as useful but not completely canon.

Hopefully path of the renegade should be good from the background point of view as it's authored by Andy Chambers, certainly he used to be part of the studio, not sure if he still is or not.

While I hate to say it, Goto wouldn't be entirely wrong having Chaos Marines in Razorbacks. After all, the races have things outside what is strictly listed in their codex, or do the traitors never loot Imperial gear? Does it make Soul Hunter a worse book because it has a sane Chaos Dreadnought in it?

I really have to side with the "everything is canon, except when it isn't" group. 40k has too many contradictions, retcons and authors with different viewpoints to ever have a single list of what's canon and what isn't. For example, while most people seem to be saying "Codices are canon, BL isn't (well, not as much)", I do my best to disregard the background from the Grey Knight codex, choosing to focus more on older sources about the Grey Knights, or the upcoming book from A D-B.
Similarly, I completely disregard The Outcast Dead from the Horus Heresy series, because of the sheer number and scale of pointless and illogical retcons in it, that served no point other than to screw up the timeline. Depending on what Path of the Renegade is like, I may or may not treat it the same way.

40k canon is what you choose it to be. There are numerous immutable facts, like the Emperor being on the Golden Throne, or that Yarrick fought against Gazhkull on Armageddon, and so on, but within that framework we are free to do as we see fit.

Put me down in the Lady Malys/Burning Eye camp. You can never be sure, but that's relatively as close to the best answer as I think you can get.