Upcoming signed offerings from Powell's:
Jonathan Evison,
 This Is Your Life, Harriet Chance! Signed Edition (Algonquin)
Print run: 50,000
I really like Jonathan Evison's writing and so does 
Kirkus - they gave him a starred review, stating that this is an "Insightful, richly entertaining look at a woman who, very late in the game, finds that life remains full of surprises." 
Amy Stewart, 
Girl Waits With Gun (Houghton Mifflin Harcourt)
Print run: 100,000
Stewart is best known for her non-fiction titles (most recently, 
The Drunken Botanist and 
Wicked Bugs). This is her foray into fiction and is receiving good reviews - stars from 
Kirkus and 
Publishers Weekly.
Salman Rushdie, 
Two Years Eight Months and Twenty-Eight Nights (Random House)
Rushdie's latest offering is a twist on 
One Thousand and One Nights. Both 
Kirkus and 
PW give it a starred review. PW states, 
"In his latest novel, Rushdie (Joseph Anton) invents his own cultural narrative—one that blends elements of One Thousand and One Nights, Homeric epics, and sci-fi and action/adventure comic books. The title is a reference to the magical stretch of time that unites the book's three periods, which are actually millennia apart."
Jonathan Franzen, 
Purity (Farrar, Straus and Giroux)
Print run: 350,000
Franzen's fifth novel gets a starred review from 
Kirkus.
If you can find a copy of his first through third novels,  
The Twenty-Seventh City (1988), 
Strong Motion (1992), 
The Corrections (2001) in 
Fine condition for less than $50, snap 'em up. (All the better if they're signed).
Christopher Moore, 
Secondhand Souls
Print run: 250,000
I don't really know the collectibility of Christopher Moore. I just like him. Some of his work is redundant, but I loved 
A Dirty Job, and 
Fluke, and the 
Bite Me series... not to mention 
Lamb.
Paolo Bacigalupi, 
The Water Knife
I highlighted this signed edition previously - Barnes & Noble was selling it with a tipped in signature page. I can't tell, based on the description, if this is from the same signed edition, or if Bacigalupi actually did a signing for Powell's.
For a larger list of their signed offerings, go 
here.