Jack Yeovil is the pen name of Kim Newman, horror film authority and film critic who occasionally writes fantasy and horror novels. He wrote some books for Warhammer at the turn of the 90s and hasn't really bothered to get back in it since. The books he did write are of ancient canon that has been retconned a million times over. What makes his works stand out, though, is that even despite his main character Genivieve being accused sometimes of being a Mary Sue (it's really divisive among the fanbase if she is or isn't) the books are intriguing character dramas instead of massive war plots, and are considered very well written to the point where they remain extremely popular, often being included on lists of best Warhammer fiction.
Drachenfels was first published back in 1989. It occupies a bizarre place in the Warhammer canon, suffering from the huge, setting-wide case of retcons that followed it. Constant Drachenfels, the titular villain and proto-Nagash, hasn't been mentioned in the background for years, having long since been displaced by Nagash as the setting's necromantic Big Bad. Details like goblins being willing to work for humans and vampires mingling relatively openly in Imperial society (indeed, the main character is a vampire who is personal friends with Karl Franz) will look like massive errors to anyone who got into the fandom any time after the early nineties. Newman's main character, the Vampire Genevieve Sandrine du Pointe du Lac Dieudonné, even appears in Newman's other non-Warhammer fiction.
Drachenfels was followed by the highly entertaining Jack the Ripper-style novel Beasts in Velvet, and the short story compendium Silver Nails. Thing is, people really like these books, so Games Workshop keep it in print and other authors give it shout outs on a regular basis - for example, his castle regularly appears on GW maps by name. They are worth reading alone for the complete difference in tone they are, compared to the usual grimdark constant war fare you get with most WHFB stuff.
In 2014, with the publication of the new campaign The End Times, fans of these books can rejoice as Geneveive and Drachenfels are confirmed as canon and the books are rumored to be getting updated by GW and Newman to fit into the modern canon.
The Genevieve series is confirmed to be reprinted as part of Black Library's Warhammer Horror imprint, and for the first time under the name of Kim Newman.