Unbroken is the fourth and final book in the Outcast Season series and in the Weather Warden time line runs alongside the last book Total Eclipse. Rachel Caine's Weather Warden and Outcast books are some of my favourite urban fantasy stories and they are ones I would highly recommend but you should really read the books in order, starting with the Weather Warden series and then following with these books. You could read the Outcast series without having read the other one first but you'd be missing out on a lot of world building and some fab characters so I wouldn't recommend it.
I didn't actually realise that Unbroken was the last book about Cassiel until I started reading it and I have to admit that I'm really sad to reach the end of the journey. The world Rachel Caine has created with both Djinn and Wardens who have the power to control the elements has been a fascinating one to spend time in and I'll miss it now it's over. Still, if you're going to come to the end of a series you want it to end on a high and that is certainly the case here.
I have really enjoyed watching Cassiel change throughout the series. As one of the old Djinn she started off as very cold and unfeeling but the longer she has spent living as a human the more she has started to show her emotions. Having said that she hasn't completely lost her original way of looking at things and she isn't afraid to make difficult decisions if it means achieving her goals. I do love the way she has come to care for Luis and Ibby though and the way they have bonded and become a little family in their own right has been really sweet. Ibby has been through a terrible ordeal and things get even harder for her in this instalment - it was quite heartbreaking to see some of the things she goes through here.
Anyone who has read Total Eclipse will recognise part of the story and I found it really interesting to see the same events but from the opposite angle. That is only a small part though because for the majority of the book Cassiel's journey is in a very different direction to Jo's in Total Eclipse. I loved getting to see more of the Djinn, spend some time with David, Jo & Lewis and of course find out if Cassiel would be able to stop Pearl. The stakes have really been raised in the last few books of both series and I was on the edge of my seat waiting to see how things would play out. I would highly recommend both of these series and I'm sure I'll be re-reading them myself at some point in the future.