I think this is my favorite in this series, and I think it contains enough incluing that if you haven't read the others, you can start with this one and only miss a little bit. The first one was very romance (little-r!) focused, the second one war, and this one is politics. So you can see why I would form an attachment.
In Without a Summer, Jane is adjusting to being Lady Vincent back in her own country, with work and family--both sides of the family--to juggle. She's never had the notoriety of being "the Prince Regent's own glamourist" to deal with before, nor has she had to help her little sister find a husband. This would be entirely enough to adjust to, except that winter has stretched into what should be spring (oh, the timing of this book!), and there's social unrest and ignorant malice to contend with as well.
My favorite parts of this book were the Coldmongers and Melody. I am utterly, completely predictable. Uprisings from a guild of lower-class magic workers? The pretty little sister turning out to have more to her than meets the eye? I'm in. And those elements turn out to be central rather than peripheral, so I gulped the book down in a matter of a few hours. Definitely recommended.