I was lucky enough to be at the top of the reserve list when Kim Harrison's prequel to The Hollows hit, meaning I'm one of the many who have now experienced The Turn.
For those who have never read The Hollows, this is likely not the place to start reading it, as the only human viewpoint character doesn't really get exposed to the Inderlanders until about 2/3 of the way through, which would likely confuse newbies trying to figure out what's going on with all the supernatural races running around.
Anyway, The Turn is set well before Dead Witch Walking, in the mid 1960's. We start with the graduation from Harvard of Dr. Trisk Cambri and Dr. Trenton Kalamak. Both are Elves, and both hate each other. (Trent is the last of his line, whereas Trisk is a "Dark Elf" with hardier genetics.) We learn that Elves in the 1960's either work in genetics or security, trying to fix the genetic curse leveled on them by the Demons and protecting their own masquerade.
Trisk and Trent used a similar doctoral thesis, in her case using viruses to insert genetic repairs, while he relied on bacterium. They also hate each other. Trisk is further hampered by the Elven predjudices of th etime mirroring that of the Human population, wherein a woman is going to wind up shuffling papers in a lab for old men. `
Trisk ends up on the West Coast working in a Human lab, the idea being that any major genetic discoveries can be passed on to the Elven Enclave. (There's a bit of alternate history here, since one point revolves around developing bioweapons that don't kill after a really ugly Cuban bio crisis.) Trisk develops the T4 Angel Tomato, which is a miracle crop able to grow in almost any environment. Her partner at the lab, Dr. Plank, develops a virus designed to make people sick for 24 hours rather than kill them.
As it turns out, other Inderland species have an interest in the research, leading to a Vampire supervisor and a Witch investor. Who in turn (along with the Were [Colonel Wolfe] and Sa'han Ulbrine) send Kal in to double check her research. Kal is accompanied by his Pixy friend Orchid, who is nowhere near as salty as Jenks in in the original series. (It should be noted here than Quen, who plays a large role in the main series is also in this one, having been hired by the Kalamaks as security. He's a friend of Trisk, and shows up intermittently in the proceedings.)
When Trisk finds out Kal is coming, assuming the worst (he's going to steal her research), she summons a demon of her grandmother's acquaintance, Algaliarept.
Anyway, to condense down a whole heck of a lot of plot, Trent in a fit of professional jealousy, modifies Plank's virus and makes it able to be hosted by the T4 Angel tomatoes. Which has the accidental side effect of making tomatoes toxic, starting off a world wide plague that eventually kills off 25% of the human population. Which starts off some unforeseen consequences, like the complete obliteration of Detroit following the breaking of the Silence by Witches and Vampires.
Eventually, we wind up in Cincinnati, wherein we see the set up of how the main series begins, but not before we come to realize there really isn't a single nice person in the book. Understandable and relatable, yes. But not a single one of them other than possibly Dr. Plank is motivated by anything other than self interest. From Trisk wanting her name on her own work, to Kal trying to bring glory to his family, to Saladan trying to make money on everything to Piscary starting his power play that eventually causes some mid series drama later on....
This is a really fun read for fans of the main series, as people we sort of know show up throughout the course of this volume. On the other hand, given how involved the main series got over 13 volumes, I ended up pulling up the character list on wikipedia to help me figure out who some of the people were with familiar names.