Or rather, details of a book we heard about more than four years ago but had since dropped off the radar. After completing Red Seas Under Red Skies back in 2007, it was announced that Scott Lynch would be writing two short stories set in the same world, The Mad Baron's Mechanical Attic and The Choir of Knives. These would be assembled into an omnibus edition, The Bastards and the Knives, for publication at a later date.
We haven't heard much about the book since, and it was assumed it would be delayed until some time after The Republic of Thieves (the third novel in the sequence) was published. According to Upcoming4.me, Gollancz have now indicated that the book will be published on 1 March 2013, with a length of approximately 100,000 words. Whilst rough cover art is provided, it's unlikely to be the final artwork as the image is a placeholder that has been used for several Lynch books in the past, and given that the art style has changed radically for the final version of The Republic of Thieves it's likely this will change as well.
As for The Republic of Thieves itself, it is still listed for a very late 2012 release but this date is - still - not finalised yet.
Note the source for the above appears to be Amazon.com, whose reliability is dubious.
Update: The release date of March 2013 is incorrect. It will follow some time after The Republic of Thieves has been published (whenever that is).