What has been said is good, however I'll be more clear: you need to find anything dark eldar related, even eldar related. That means anything, new codex, old codex, eldar codex, older editions, fluff and rules for another specialist game (battlefleet gothic for example), white dwarf entries, any expansions (apocalypse, etc...any expansion book old or new) as long as the dark eldar are mentioned you're good. Weirdly enough even pieces of fluff found in other codex are reliable sources.
The Black Library books aren't considered 100% reliable fluff, they're not canonical in fact.
Now the dark eldar trilogy does have reliable fluff, but it's mainly because of who wrote it, a guy who has worked on the rulebooks and codices, what he describes is very accurate, if you've read the fluff, you can confirm it. Now things went very bad with the dark eldar BL books, as most of them described chaos worshipping DE...not surprisingly written by unknown authors, who obviously had no idea what dark eldar were, neither did they have read the old codex ><.
I know that a lot of people read the BL books, but a lot of the old stuff was really bad, badly written, etc... things for a lot better with the years to the point where the novels are beign recognised by non 40k groups.
Like I said, not just dark eldar stuff, the eldar trilogy is also good and also has some major dark eldar fluff, the third book of the eldar trilogy: Path of the Outcast is actually half about dark eldar/corsair stuff. Several things aren't said in the dark eldar codex, but said in the eldar codex, for example: very dumb: how are eldar different from humans, etc...it's basic stuff, but not covered in our codex.
If you want to be a dark eldar fluff nerd, you'll read anything with eldar mentioned in it, stuff like codices, read several times to know pretty much by heart. Honestly you don't want to know how many times I've read all my codices,I'm talking all editions from all the races, the other day I was reading our old codex for the fun of it.
Fluff can be found far back, previous rulebook editions, supplements, small fluff pieces in white dwarfs, as far a rulebook it goes back to rogue trader, though at that time you had very lil DE fluff, but plenty of eldar and harlequin fluff. Second edition there's plenty of fluff, but technically the eldar described before the fall, some were the ancestors of the current dark eldar, meaning they're part of our fluff too, that's why rogue trader does matter, fluff about before the fall in the eldar codex does matter a lot.