Kristin Cashore's companion books, Graceling, Fire, and Bitterblue are some of the most incredible YA fantasy books I have read in the last few years. Cashore's world-building is first-rate and she has given us a cast of characters that is truly unforgettable. I loved how Bitterblue masterfully ties together the stories from Graceling and Fire, particularly towards the end of the book. All of that said, I actually had a difficult time getting into this story -- it started off very slow and it took muddling my way through several chapters before I was finally hooked and didn't want to put it down. (The slow start is what ultimately caused me to deduct half a star in the rating on my blog.)But once the story gets going, it moves along at full speed and doesn't let up or hold back. Bitterblue is much, much darker than the first two books as we take a closer look at the consequences of Leck's disastrous 35-year reign and what it is going to take to heal the kingdom of Monsea. I was left breathless by some of the dark deeds and events that are revealed. No, this was by no means an easy read, but that level of twisted detail with which Cashore wrote this tale was absolutely necessary to do the story justice. There are an abundance of new and familiar faces in Bitterblue. We are introduced to Bitterblue's four advisors, Thield, Rood, Runnemood, and Darby, their involvement in Leck's experiments during his rule, and the effect they have on Bitterblue and her own reign. She also crosses paths with small band of "truthseekers," leading to a few side plots that complicate matters for her. Perhaps my favorite new character is her cantankerous, crotchety librarian, Death (pronounced Deeth). And of course, we have the return of Katsa, Po, Raffin, and Giddon as they they travel to and from Monsea. With Bitterblue, Cashore again writes about a strong heroine -- this story is key to her growth and ultimately the truths she uncovers coupled with how she handles the discoveries is what causes her to truly come into her own as Queen of Monsea and the woman she has become.As for the rest, one of my favorite aspects of this novel was the use of mathematics and ciphers -- that provided yet another fascinating bit of intrigue to the tale. And once again we have a brewing romance, but as with Cashore's other books, the romance is not really a central theme. Interestingly, I think this practice is an important part of what makes her stories so successful and leave you wanting just a bit more. (And I mean that in a good way.)If you have not read these books, please be sure to read Graceling and Fire (in whatever order you please) before reading Bitterblue. If you've already read the first two, you will absolutely want to get your hands on Bitterblue.