Imagine it's 1879, and the American Civil War refuses to end. Imagine the technological advancement that comes from protracted wartime, the same thing that produced the first iron-plated battleships, stretching 15 years longer than our own history allowed. Imagine an industrial mining experiment gone terribly awry, releasing a mysterious undead plague across a familiar but slightly skewed Seattle.

For Cherie Priest, only one outcome is possible: "Zombies, air pirates, secret criminal societies, and mad scientists ensue!"

See, Priest has already imagined all of this for you. Newly nominated for the 2010 Hugo Award - among the top honors in the sci-fi/fantasy industry - her novel, Boneshaker, takes on a Washington trapped in territorial limbo as the government deals with a war that will not end, a Pacific Northwest cut off by the decision to route the transcontinental railroad south to California - and a zombie-ridden Seattle walled off from civilization and left for dead.

Says Priest, "My goal was to write an essentially ‘Northwestern' story, something that couldn't happen anywhere but here - even if it couldn't really happen at all ... my latent desire to explore steampunk as an author collided with my new place of residence, and the result was Boneshaker."

The novel is part western, part sci-fi, part thriller, part alternate history and part action-adventure. The genre of steampunk is, essentially, a look at the world as if serious modern technological innovation took off 50 years early, so Priest's war-driven America is filled with combat-ready dirigibles and mad, steam-power inventors.

"I've always been a big fan of Victoriana, especially the fun tech innovations of the day," Priest says.

"Seattle was destroyed by a mining machine called the ‘Boneshaker,' which released a vein of noxious volcanic gas that turned people into zombies," explains Priest of the plot. "Subsequently, Seattle's survivors walled off the city and abandoned it.  Now it's fifteen years later, and the son of the man who created the mining machine ... gears up and sneaks under the wall.  His mother is appalled. And she goes in after him."

Boneshaker has been a breakthrough novel of sorts in Priest's career, garnering her foreign rights deals and a forthcoming audiobook featuring professional geek and former Star Trek cast member Wil Wheaton.

And it will not be the end of Priest's literary relationship with the Northwest corner of this parallel world, under the blanket title Clockwork Century - the alternate history created by Priest in which Boneshaker and future novels are set.

"Clementine (May 2010) will go back east across the plains, across Kansas and over the Mississippi River into northern Kentucky," Priest tells me. "And Dreadnought (fall 2010) starts in Richmond, Virginia and works its way back to the Pacific Northwest."

Priest brings her zombie-ridden take on this increasingly popular form of fiction to Tacoma April 15, reading and signing at 7 p.m. at the Garfield Book Company, off the campus of Pacific Lutheran University.

Should you attend, don't be surprised to see a genre costume or two.

"At least a handful out of every group (show up in brass painted goggles). Some people go all out, which is amazing," says Priest. "I love playing dress-up myself, so it always tickles me when people show up to events having gone to a lot of trouble to look awesome."

[Garfield Book Company, Cherie Priest reads and signs Boneshaker, Thursday, April 15, 7 p.m., free, 208 Garfield St S., Tacoma, 253.535.7665]