I don't know what it is, but I always find the books I like the most the hardest to review. 
 This was one of them. 
 This is one of the most imaginative series (books 1-5) that I have come across. 
 The world building and lore are just fantastic and I literally devoured every bit of information I got on them. 
 I love that good characters do evil things and evil characters do good things in this book. 
 There are tons of questions to be asked and answered, and the best thing about this is that I couldn't guess many of the answers right (view spoiler)[I guessed that V'lane was really Cruce though, yay (hide spoiler)], and that was soooo good. 
 I also really enjoyed the dialogues. 
 V'lane: Are you busy tomorrow MacKayla ? 
 Barrons: She's working on old texts with me. 
 V'lane: Ah. Old texts. A banner day at the bookstore. 
 Barrons: We're translating Kama Sutra...with interactive aids. 
 I felt like re reading this series the moment I finished it and go clue hunting. 
 Books 1-5 are totally recommended.