There probably isn't one
There are several hundred published Warhammer Fantasy books from the Black Library. Of the 153 that I was able to search, 64 contain the word (or strictly speaking, string) "ghoul".
"No actual ghouls" means that the string ghoul appears, but ghouls themselves aren't mentioned. This basically means words such as ghoulish.
The Hour of Shadows: No actual ghouls
The Claws of Chaos: Ghouls mentioned, but not seen
Age of Legend: Ghouls seen, but no transformation
Ancient Blood: Ghouls seen, but no transformation. This one has humans closely associated with ghouls, though, and one such character makes medicine that secretly contains human organs, strongly suggesting that she is turning her people into the ghouls with which they associate.
Archaon: Everchosen : No actual ghouls
Archaon: Lord of Chaos: No actual ghouls
Blighted Empire: Ghouls mentioned, but not seen
Blood for the Blood God: Ghouls mentioned, but not seen
Bloodborn: Lots of ghouls seen, but no transformation
Bloodforged: Ghouls mentioned, but not seen.
Bloodsworn: Ghouls seen, but no transformation.
Charnel Congress: Ghouls seen, but no transformation
Curse of the Necrarch: Ghouls seen, but no transformation
Dead Winter: Ghouls mentioned, but not seen
Deathblade: A Tale of Malus Darkblade: No actual ghouls
Dominion: Ghouls seen, but no transformation
Empire: No actual ghouls
Ghoul King: Conqueror of Worms: Ghouls presumably seen, but according to the questioner, there are no ghoul transformations.
Ghoul King: Empire of Maggots: Same
Gilead's Blood: Ghouls mentioned, but not seen
God King: Ghouls seen, but no transformation
Gotrek & Felix: Kinslayer: Ghouls mentioned, but not seen
Gotrek and Felix: City of the Damned: Ghouls mentioned, but not seen
Gotrek and Felix: Road of Skulls: Ghouls mentioned, but not seen
Gotrek and Felix: The Anthology: Ghouls seen, but no transformation
Grey Seer: No actual ghouls
Grimblades: No actual ghouls
Hammers of Ulric: Ghouls seen, but no transformation
Harbinger: No actual ghouls
Headtaker: No actual ghouls
Inheritance: Ghouls seen, but no transformation
Knight of the Blazing Sun: No actual ghouls
Marienburg's Stand: No actual ghouls
Master of Death: Ghouls mentioned, in particular the process of creating them. No transformation
Master of Dragons: No actual ghouls
Master of Mourkain: Ghouls mentioned, but not seen
Nagash the Sorcerer: No actual ghouls
Nagash the Unbroken: Ghouls seen, but no transformation
Neferata: Ghouls seen, but no transformation.
Picking the Bones: Ghouls mentioned, but not seen
Plague Priest: No actual ghouls
Retribution: Ghouls seen, but no transformation
Sigmar's Blood: Ghouls seen, but no transformation
Skarsnik: Ghouls seen, but no transformation
Sword of Vengeance: Ghouls seen, but no transformation.
Temple of the Serpent: No actual ghouls
Thanquol's Doom: No actual ghouls
The Blades of Chaos: No actual ghouls
The Curse of Khaine : Ghouls mentioned, but not seen
The End Times: The Bone Cage: Ghouls seen, but no transformation
The End Times: The Fall of Altdorf: Ghouls seen, but no transformation
The End Times: The Lord of the End Times: Ghouls seen, but no transformation
The End Times:The Return of Nagash: Ghouls seen, but no transformation
The End Times: The Rise of the Horned Rat: Ghouls seen, but no transformation
The Fangs of the Asp: Ghouls seen, but no transformation
The Great Maw: Ghouls mentioned, but not seen
The Last Man: No actual ghouls
The Red Duke: Ghouls seen, but no tranformation
The Serpent Queen: Ghouls seen, but no transformation
Vampireslayer: Ghouls seen, but no transformation. However, a ghoul is mentioned to possibly be someone known to the townspeople, indicating the transformation can be rapid.
Vermintide: Ghouls mentioned, but not seen
Warrior Priest: No actual ghouls
Wolf of Sigmar (Time of Legends): No actual ghouls
Zombieslayer: Ghouls seen, but no transformation
The preceding list comes from these series, as well as several standalones:
- Chaos
- Dwarves
- Elves
- Empire
Gilead
Gotrek and Felix
- Time of Legends
- Vampires
Warhammer Heroes
Skaven
- Storm of Magic
- The End Times
I also checked all the listed Warhammer short stories that I could:
None of those whose description I could find has ghouls as a main subject, and as such are fairly unlikely to depict the process of a ghoul transformation, or discuss the details thereof.
Finally, of course, I searched online extensively for Warhammer novels containing ghoul transformations, and found none.