In terms of content, these are really original fantasy stories with characters we can care about undertaking risky missions and developing their personal strengths along the way. While the main characters from each book - Katsa, Fire, and Bitterblue - are very different heroines, they are all characters I could relate to and easily became invested in. The books do stand alone and don't have to be read in order or together at all, but I enjoyed reading them in the order in which they were published. There's action, adventure, nobility, loss and romance but in very original settings that kept me captivated throughout.
The final book - Bitterblue - loses a little bit of the action of the first two and concentrates a little more on the politics of Bitterblue's kingdom than on action and adventure, but there's still a sufficiently interesting plot, romance, adventure and wrapping up of character's stories from the other books. So if you bear in mind before you read it that it's a little more cerebral and political than the other books before you start reading it, you shouldn't be disappointed.