Type 0.00 in the search area which should result in thousands of hits. Type 0.00 Mystery to get only the mystery, etc.
You can also look on Inkmesh.com, they list free books.
Oh, and if you get a message back that it couldn't find anything with 000, then click on the books menu item to get to the main page, then type 0.00 and change to Nookbooks. I have no idea why, at times, it returns nothing, but I've found that starting from the main page seems to fix it.
Or look at Doug's post, the master of finding the free books, he's constantly updating it.
And don't forget the unbound blog on friday, for the free friday highlighted book.