The Comic "Deathwatch", the Black Library novel "Deathwatch" and the Black Library novel "Deathwatch: Overkill".
Audiobook "Mission: Purge".
Ciaphas Caine books - genestealer cults are a repeated opponent in quite a few of them. For the Emperor, and the Emperor's Finest for definite.
So can someone explain to me how the army of Genstealer Cult will look like? Is it like some Genstealer weird humans + regular guys from Imperial Guard + Tyranids?
Short version:
Tyranids send out genestealers as a 'vanguard' to find inhabited worlds. Space hulks, things like that.
One or more genestealers makes it to a populated planet.
Genestealers - mature, full-grown ones (like you get in the
GSC codex, not the quick-grown throwaway things the tyranids use as shock troops) - can infect and/or mind control whatever species they hide within the society of.
The cult tends to pretend to be a benevolent religion, or charity, or worker's association - whatever will let it survive in the shadows with plenty of members and no official crackdown. Humans infected give birth to hybrids. Hybrids get more human-looking with each generation.
Eventually (I think 5th generation?) a genetic 'timebomb' goes off, and the babies born to the cult are all switched back to purestrain genestealers, and they, and all the suborned humans and hybrids, try to take over the world - at the same time sending out a telepathic message to the tyranids "
all you can eat buffet over here!!!"
The resulting army is mobs of genestealers (and hybrids), plus normal workers who've siezed guns to defend what they perceive as their gods, plus (normally) planetary defence forces or guard stationed on the planet (because the cult tends to try very hard to infect and corrupt any regular army to improve its chances).
In theory you can ally in some tyranids but you don't need to. A lot of the time the stealer cult takes over the world all by its lonesome and sits around happily waiting for god to arrive (and dissolve them all in a digestion pool, but hey, that's organised religion for you).