IMO, though, there are a few tiers of sourcing for lore in re fluffy army compositions:
Top -
GW codices are best for general feel of an army and relevant
FW books are best for nitty-gritty "how many lasguns per company" details (older is generally better, but that's partly codices shifting away from a lore-heavy layout to more of a product catalog format, partly an artifact of the old writers leaving over time, and partly me being a grog). I'd say Lexicanum lives here due to its encyclopedic (duh) knowledge and use of
GW sources. That said, as beast_gts said (and as with any wiki), you should try to find and read the materials Lexicanum uses as sources rather than using Lexicanum by itself.
Penultimate - White Dwarf (and Index Astartes for Space Marines) can also be invaluable, if harder to source and sift through. Also, the
BRB blurbs, campaign books, and other non-codex gaming material.
High-mid - Researching the real-life inspirations of the army, should one exist. Some educated guessing can help fill gaps here - eg, a steppe-nomad-raider force in
40k might take Land Speeders or Vypers or Valkyries rather than horses (the intent is "rush in, hit hard, get away", so anything in the roster that helps do that would be fair game). Even in cases where there may not be an actual inspiration, some thought to the themes present can be helpful as well ("Viking" is a seasonal job, not a force composition, but you can still use it to inform your unit selection - Nords were big on death in combat and taking your wounds to your front, so they'd be less likely to take sneaky snipers and more likely to rely on melee units charging straight down the center).
Low - Things like video games and Black Library. They can be OK, but they're much more scattershot in quality, even more subject to the whims of narrative convenience and biases (or ignorance *cough* C.S. MULTILASER *cough*) of the author, and there's no guarantee a given bit of fluff from a novel will ever be referenced again (outside Lexicanum).
Questionable - YouTube, Reddit, fan forums. There're some great resources out there, but there's also a lot (a
LOOOOOOOOOOOOOOOOOOOOOOOOT) of misreading, misremembering, fanon, headcanon, laziness, lies, stupidity, and other assorted bullgak, too.
Unimpeachable - DakkaDakka. Everyfing posted 'ere 'as been vermifi... um, vreif... SOME BRAINY GIT CHECKED
IT.