here (for the UK) and here (for the US).
The kindle version is here (if you're in the UK) or here (in the USA).
There will be versions at Lulu and DriveThruFiction, but they haven't appeared yet - I'll post when they appear.
It seems to have been a long haul to get Sorrel to this stage in her adventures, but I think it's been worth the wait (of course, I would say that, wouldn't I? ;-))
Sorrel Snowbound starts shortly after the end of Sorrel in Scarlet. The city of Daryan is under attack by the lloruk, and Sorrel and Wrack have travelled there from Tolgrail to offer support and aid. In the midst of the struggle Wrack is captured and transported to Luthvara, a lloruk city. Sorrel is determined to get him out of there - but fears the sophisticated sorcery of the lloruk. She needs an edge - something that the lloruk have not faced before. Her best chance is to return to the surface and gather knowledge and hardware the lloruk will not expect.
But the lands of the surface are (in their own way) even more dangerous than the scarlet depths of the Chasm...